The Progress of Astronomy: a Lecture

Delivered in the Duke of Edinburgh Theatre, on the 25th July, 1870,

By

R. L. J. Ellery, Esq.,

Government Astronomer.

The Hon. Sir James M'Culloch presided, and introduced the Lecturer, who said:—

In the year 1676, Charles the Second appointed Flamsteed to be the "King's Astronomical Observator," at a salary of £100 per annum. The duties of the "Astronomical Observator" were thus set forth:—"He is to forthwith apply himself with the most exact care and diligence to the rectifying the tables of the motions of the heavens and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting the art of navigation."

At this time the Greenwich Observatory had just been erected, and as the building of the observatory and the appointment of Flamsteed mark an epoch in the history of Astronomy, I propose to take this, one of the few good acts which embellished Charles the Second's reign, as the starting point in my lecture.

To give us a fair start, however, it will be well to briefly review the state of astronomical knowledge at this period, in order that "we may more readily estimate the progress subsequently achieved.

You will remember that, long before this, the older theories of the universe had been cast aside, and the solar system as we now know it firmly established. The rotation of the earth had been proved, though even at that time questioned by some philosophers. The planets Mercury, Venus, Mars, Jupiter and Saturn were well known, and Kepler had enunciated his three great laws of planetary motions, whose correctness Newton afterwards tested, proved, and from them deduced his theory of page 2 gravitation. These laws were—1st, That the planets move in ellipses, having the sun for one of the foci; 2nd, They describe equal areas in equal times; and 3rd, That the squares of their periodic times are proportional to the cubes of their mean distances from the sun.

The last law gives us the means for deducing the relative distances of the planets from the sun from the time they occupy in making a complete circuit of their orbits; for example—if the sun's distance from the earth be taken as the unit, that from Venus will be found to be seven-tenths of this unit, and Mars one and a-half times.

The four satellites of Jupiter had been discovered by Galileo, and one of Saturn by Cassini; Huygens had also discovered the ring surrounding Saturn; and it had been shown that Venus, Mars and Jupiter rotated on their axes. Telescopes had been invented since 1608; Gregory made his first reflecting one in 1663, and Newton his in 1669. Clocks and spring watches had also been introduced. Charts of the heavens had been constructed, as well as tables of the approximate motions of the moon and other heavenly bodies; and successful efforts had been made to determine with more precision than had before been attempted the figure and dimensions of the earth, by the measurement of certain lengths of meridians by Norwood, Picard and La Hire.

Before referring to the astronomical instruments in use at this time, it will be well to point out that they may be classed under three heads. First, those which enable us to bring distant objects near to us, so as to render their form, features, and peculiarities visible: second, instruments by which the angular separation of one body from another can be measured; and third, instruments by which time can be measured.

The first class simply comprises telescopes of various forms; the second class depends upon the subdivision of the circle into parts—into degrees, minutes, and seconds. One of the oldest astronomical instruments of this kind was the astrolabe, a divided circle, about whose centre revolved an arm, carrying sights or pins; its extremities pointing to different divisions on the circle, as the sights were pointed in different directions. The complete circle afterwards gave place to portions of a circle more accurately graduated, such as a fourth or sixth part of a circle called quadrants and sextants, forms of which have continued in use up to the present day. The third class, for measurement of time, you are familiar with in the form of the exquisite clocks and watches which are now so common. At the time I am now speaking of, however, although the old methods of measuring time by clepsydras or water clocks, sand glass or "rush lights," had been entirely superseded for astronomical purposes, the clocks and watches were of a most homely description.

During the observatorship of our first Astronomer-Royal, Flamsteed, the instruments used consisted principally of large sextants and quadrants, which had already been greatly improved by the substitution of telescopes for the simple tubes and sights hitherto used, and by the more careful graduations of their circular arcs. These instruments were very large compared to those of the same class now in use, and frequently occupied the whole wall of a large room, or even the whole side of a large house—as in the case of Tycho Brahe's mural quadrant at Copenhagen—and it is somewhat amusing to compare these gigantic instruments with this page 3 small pocket sextant, more especially when we consider that this little apparatus is capable of giving better results than its big forefather at Copenhagen.

The reluctance exhibited by astronomers and navigators, at this time, to adopt new inventions, was remarkable. We find that the substitution of "optical glasses," or telescopes for plain sights in the large sextants and quadrants, was strongly opposed by some of the most celebrated observers. Hevelius, for instance, in writing concerning his astronomical observations, about this time, says:—"But this I am convinced of, that if I had undertaken the business by means of telescopic sights, I must not only have wasted many years in fruitless examinations, but I should doubtless have been disappointed of my hopes, and that on various occasions, not necessary here to mention."

It is worthy of note that, in the appointment of Flamsteed, he was directed "to set himself to the rectifying the motions of the heavens and the places of the fixed stars, so as to find out the so much desired longitude of places." One of the principal objects of the establishment of Greenwich Observatory was to facilitate navigation—an object that has been faithfully kept in view and assiduously followed up from the time of its foundation to the present day.

The progress of the art of navigation had for a long time been retarded, from the difficulty of determining the longitude at sea. Several methods had been propounded, but owing to the insufficient knowledge of the true places and motions of the heavenly bodies, and defective instruments, it had not yet been possible to put them in practice.

These methods resolved themselves into observations for determining the moon's motions with respect to the stars, or to transport of the true local time of the point of departure from place to place; the first requiring angle-measuring instruments and tables of the moon's place and motions, and the second accurate time-keepers or chronometers.

Longitude is, in fact, the difference of local times between any two places at the same moment; so that if we can carry a clock or watch set to, and keeping true Melbourne time, to Adelaide, we should find that it did not correspond to Adelaide time. "When the watch showed twelve o'clock, it would only be 11h. 34m. 27s. there. This difference is a measure of the difference of longitude; multiply it by 15, and it gives the difference in degrees, minutes and seconds. Melbourne time is faster than that at Adelaide, and therefore Melbourne is east of Adelaide 6° 23′ 15″. If, instead of Adelaide, we took the watch to Sydney, we should find the difference to be 25 min. 56 sec., but the other way. Sydney time would be faster than Melbourne, showing Sydney to be east of Melbourne.

This will give you a notion of what is meant by longitude: every method for obtaining it resolves itself into ascertaining the difference between the times at the place whose longitude you wish to determine, and that at the standard or starting point, for which Greenwich is now almost universally used. The time at your starting point (or Greenwich) can be kept by means of good timekeepers, or it may be found by careful astronomical observation of the moon's place, and "comparing this with tables showing what her place will be at certain hours of time at the starting point; the local time is readily determined by observation and page 4 simple calculation—thus the differences of local times, and hence the longitude, can be found.

But, as I have already stated, neither the instruments available nor the knowledge of the moon's motion were sufficiently accurate to allow of these methods being satisfactorily practised in the year 1675; hence the great and principal object for the foundation of Greenwich Observatory—"the finding out the so much desired longitude of places."

The method of finding longitude by observing the moon's place, by getting her distance from known stars, was proposed many years before, but could not be carried into practice on account of the inaccurate lunar tables. The method of transport of time had already been tried by Huygens; and it was during his first trials to overcome the difficulty presented to the good going of his timekeepers by the violent motion of a ship at sea, that he fell upon the discovery of the isochronous spring, with which he made afterwards his pendulum watches, and which is the same that is now used in our watches and chronometers. Huygens partially succeeded, but found that to render his timekeepers sufficiently reliable, it would be necessary to devise some greatly improved mechanism by which the irregularities in them, brought about by changes of temperature, could be overcome.

It will be interesting to show with how much more precision the longitude could be determined with Huygen's primitive and uncompensated "pendulum watch" (as he styled it,) than by the methods already in use, and to do this I cannot do better than give you an abstract from a paper communicated to the Royal Society of London in the year 1665, by a Major Holmes. He states that in one of his voyages to Guinea, he had given into his care, by some of the grand promoters of navigation, two of M. Huygens' "pendulum watches,"—that after leaving the coast of Guinea, and "being come to the Isle of St. Thomas, under the line, he adjusted his watches, put to sea, and sailed westward seven or eight hundred leagues, without changing his course; after which, finding the wind favourable, he steered towards the coast of Africa N.N.E., but, having sailed upon that line about two or three hundred leagues, the masters of other ships under his conduct, apprehending that they should want water before they could reach that coast, did propose to him to steer their course to the Barbadoes, to supply themselves with water there. The Major having called the master and pilots together, and caused them to produce their journals and calculations, it was found that those pilots differed from the Major in their reckonings, one of them eighty leagues, another about a hundred, and the third more; but the Major judging by his pendulum watches that they were only some thirty leagues distant from the Isle of Fuego, which is one of the isles of Cape Verde, and that they might reach it next day, and having a great confidence in such watches, resolved to steer their course thither; and having given order so to do, they got, the very next day about noon, a sight of the said isle of Fuego, finding themselves to sail directly upon it, and so arrived at it that afternoon as he had said."

Another method for finding the longitude had also been proposed. Jupiter was known to have four satellites, and it had been observed that these were frequently eclipsed, disappearing suddenly by entering page 5 within the planet's shadow as they revolved around him. It was proposed that these occurrences, the times of which could be predicted with moderate precision, should be observed at sea, and the local time, compared with the predicted time at the starting point. To observe these eclipses a steadily supported and powerful telescope is required, and although this might be secured on land, it could not on shipboard—the method, therefore, was impracticable for use at sea.

I think I have now briefly touched upon the most important points in connection with the state of astronomy at the date I start from. But before we leave our first Astonomer-Royal Flamsteed, I will give you a rather amusing anecdote concerning him, taken from Reliquœ Hearniainœ, and given in Chambers's Book of Days:—"He was weakly and unhealthy in childhood. His father, a maltster at Derby, set him to carry out malt in a brewing pan, which he found a very tiresome way of effecting his object; so he set to and made a wheelbarrow to carry the malt. The father then gave him a larger quantity to carry, and young Flamsteed felt the disappointment so great that he never after could bear the thoughts of a wheelbarrow. Many years after, when he reigned as Astronomer-Royal in the Greenwich Observatory, he chanced once more to come into unpleasant relations with a wheelbarrow. Having one day spent some time in the Ship Tavern with two gentlemen artists of his acquaintance, he was taking a rather ceremonious leave of them at the door, when, stepping backwards, be plumped into a wheelbarrow. The vehicle immediately moved off down-hill with the philosopher in it; nor did it stop till it had reached the bottom, much to the amusement of the bystanders, but not less to the discomposure of the Astronomer-Royal."

Whilst our Royal Observator was "applying himself with exact care and diligence rectifying the places and motions of the heavenly bodies," and constructing his star catalogues and charts, the famous Halley took a voyage to St. Helena in order to obtain a catalogue of southern stars, which could not be seen by northern astronomers. It was he who, out of compliment to Charles II., formed a new constellation of some southern stars which he called Robur Carolinum, or Charles's Oak, in commemoration of the well-known tree in which the King (then Prince Charles) took refuge after the Rattle of Worcester.

Whilst at St. Helena. Halley witnessed the transit of Mercury across the sun's disc. You will remember that both Mercury and Venus are inferior planets, having their orbits between us and the sun; they are, therefore, liable in their orbital motion to pass between the earth and the sun, appearing to move across his disc like a small round black spot; and such occurrences are called transits.

That such phenomena did sometimes occur had long been known, and the times of some had even been predicted by Kepler—a transit of Mercury in 1631 for instance—which was the first really observed with certainty. A Parisian astronomer named Gassendi thus quaintly describes the event:—"The crafty god had thought to deceive astronomers by passing over the sun a little earlier than was expected, and had drawn a veil of dark clouds over the earth in order to make his escape more effectual. But Apollo, acquainted with his knavish tricks from his infancy, would not allow him to pass altogether unnoticed. To be brief, page 6 I have been more fortunate than those hunters after Mercury who have sought the cunning god in the sun; I found him out, and saw him where no one else had hitherto seen him."

The first time the transit of Venus was observed was in 1639 by Mr. Horrox, a clergyman in Lancashire. This is what he tells of it. He began his observations at sunrise, and continued them to the hour of church service (it was Sunday.) As soon as he was again at leisure—that is to say, at 3.15 p.m.—he resumed his labours, and, to quote his own words:—"At this time an opening in the clouds, which rendered the sun distinctly visible, seemed as if Divine Providence encouraged my aspirations; when, O most gratifying spectacle, the object of so many earnest wishes, I perceived a new spot of unusual magnitude, and of a perfectly round form, that had just wholly entered upon the left limb of the sun, so that the margin of the sun and spot coincided with each other, forming the angle of contact."

The occurrence of the transit of Mercury, which he observed at St. Helena, suggested to Halley the possibility of using like occurrences for the solution of one of the greatest problems in astronomy, and the basis of all celestial measurements—the distance of the sun from the earth.

The means of ascertaining the relative distances of the planets from the sun had been given to astronomers by Kepler's Third Law, so if the absolute distance of any one could be found, the rest would follow. Halley perceived that the parallax of the planet might be found by the apparent alteration of the position of its path across the sun's disc, due to observers being placed at widely different parts of the earth's surface.

By the word parallax, I mean the angle that would be subtended, at the planet, by the earth's diameter. Determining the parallax is equivalent to finding the distance of an inaccessible object by means of a triangle, the base of which, in this case, is the earth's diameter, the two angles being obtained from observation. Now, Venus being nearer to the earth than Mercury, her parallax is greater, and consequently the separation of her apparent path across the sun would be greater in proportion. The transits of Venus, therefore, were considered by Halley to present far more favourable conditions for the desired determination than those of Mercury.

The transits of Mercury are comparatively frequent: for instance, there will be thirteen in the present century, while those of Venus occur very seldom—not more than twice in a century,—and Halley scarcely dared to hope he would live long enough to put his suggested method in practice, for it was then the year 1677, and there would be no transit of Venus till 1761, a space of eighty four years. He, however, addressed to future astronomers a most earnest exhortation that such "precious occasions should not pass unprofited," and urged them to unite all their efforts to deduce on this and similar occasions one of the most important elements of our system, the distance of the sun. How this request has been fulfilled, we shall presently see.

The determination of the parallax of Mars—the planet next to the earth in order of distance from the sun—has also been used for finding the sun's distance. This planet being "superior," never transits the sun; a different mode of observation is therefore necessary. When in oppo- page 7 sition, its position with reference to that of well-known fixed stars is measured at two widely different parts of the earth's surface.

Halley was the first astronomer to actually compute the orbit of a comet, and predict its return—(the method for doing this had already been indicated by Newton, in 1680, after the appearance of the great comet of that year.) Among the orbits of about twenty-four comets, which he calculated, he found those of 1531, 1607, and 1682 so nearly alike—in each case the inclination to the ecliptic was about 17°, the perihelion distance about 48,000,000 of miles, and the motion retrograde, while the other elements were equally similar; and, moreover, just about seventy-five years had elapsed between each successive appearance—that he at once concluded they were one and the same comet returning periodically to perihelion. He felt so confident of this that he predicted it would return again in 1758. His age at this time—1705—was forty-nine; he did not expect to reach the age of one hundred and two years; he therefore appealed to posterity that if the comet should really return about that time, not to lose sight of the fact that the prediction of such a result was due to an Englishman. As the period drew nigh, astronomers throughout the civilised world were on the watch to test the value of his calculation and predictions.

On the 25th December, 1 758, a farmer and amateur astronomer in Saxony, named Palitsch, discovers a comet; it was Halley's comet. It came back again in 1835 (and was the first comet I ever saw,) and will probably do so again in 1910. This fulfilled prediction marks another epoch in astronomy;^* and, remembering Halley's appeal, posterity of all nations has not, and is not likely, to forget that this great triumph was Halley's, and that Halley was an Englishman.

We will now pass over the next fifty-eight years, which brings us to the year 1800. In these fifty-eight years astronomy had not slept. Flamsteed and his successors—Halley, Bradley, and Bliss,—as well as various celebrated Continental astronomers, had so faithfully attached themselves to the work for which the Greenwich Observatory was established, that the places and motions of the moon, stars, and planets, Mere now known with very considerable precision, and reliable tables of these were published. The art of navigation had therefore advanced considerably, and the great problem of obtaining the longitude at sea had been, to a great extent, solved. All these astronomers, however, had now gone to rest, and Maskelyne was "Royal Observator." The immortal Newton had closed his great career in 1727, and the glorious results of his labours placed Astronomy the highest anion" the exact sciences, and gave an impulse to it and other branches of Natural Philosophy which will never cease to be felt.

One of our greatest of the Greenwich, and of all astronomers, had come and gone in this interval. Bradley succeeded Halley in 1742, worked well for twenty years, and died in 1762. It was Bradley who discovered the aberration of light, and the nutation of the earth's axis—an example of the one being the amount by which the apparent position of a star differs from its real one, in consequence of the combined motion page 8 of the spectator (resulting from the earth's rotation), and of the light emanating from the star; and the other a small irregularity in the earth's motion, caused by lunar gravitation—both affecting the positions of heavenly bodies, as determined from observation, to an extent, if uneliminated, that would render them very inaccurate. Bradley also initiated and prosecuted a system of observation, which will compare in precision with that of the present day. With Bradley's observations commenced a new era in astronomy.

In this interval, too, Dollond invented the Achromatic Telescope, and Hadley the Nautical Quadrant. Harrison also gains the parliamentary reward for the perfection of his chronometer. The Nautical Almanac was first published in 1767. The transits of Venus, both of 1761 and 1769, were observed. Herschell (Sir W.) discovers the new planet Uranus, and erects his great forty-feet reflecting telescope at Slough, near Windsor. Large observatories had been erected at Berlin and St. Petersburg. Dollond's new invention of making telescopic object glasses achromatic, rapidly led to a very great improvement in all classes of astronomical instruments; and the old mural quadrants were replaced by transit telescopes and mural circles. Abbé La Caille had been to the Cape of Good Hope, measured an are of the meridian there, observed and catalogued a large number of southern stars with great precision; and Sir William Herschell had discovered that the Milky Way was separable into stars.

I will now draw your particular attention to three of these "signs of progress," and in the first place show how Halley's suggestions with respect to the future transit of Venus were attended to.

The first transit after Halley's death was in 1761. To observe it, astronomers were sent out to different parts of the globe by the English, French, and other European Governments; and Maskelyne, who afterwards succeeded Bradley at Greenwich, went to St. Helena. With one exception, however, all these observers were unfortunately more or less disappointed; cloudy weather appeared to prevail at every one of the stations selected, except at the Cape of Good Hope, where Mr. Mason observed all the phases successfully. At Greenwich and several other places in England it was also satisfactorily observed. The parallax of the sun resulting from these observations was eight seconds and a half, which would give the sun's distance as about 96,000,000 miles.

The next transit took place in June, 1769; and it was on this occasion that George III. despatched, at his own expense, a well-equipped expedition to Tahiti, under the command of that celebrated navigator Captain Cook. The phenomenon was successfully observed in both hemispheres, and the parallax resulting was 8″ 58—equivalent to a distance of 95,023,000 miles.

We find, therefore, that Halley's request was not forgotten, and that the results fully justified his anticipations. The great problem of the sun's distance was far nearer its solution than it ever had been; and this determination was accepted as perfectly trustworthy until later years, when circumstances arose indicating that it required even further correction. To this I shall presently refer.

The next transit of Venus will take place four years next December (December 1874,) which will be followed by another in 1882.

The improvements in the methods of obtaining the "much desired longitude" during the period under review are worthy of remark, and a few moments' attention: for on two or three of them depends, in a very great measure, the state of almost perfection to which the art of navigation has now arrived. I refer to the invention of Hadley's Quadrant (1731,) Harrison's Chronometer (1765 and the publication of the Nautical Almanac in 1767.

From what I have already told you concerning the methods of obtaining longitudes at sea, you will understand that at my last stage the great desiderata were—more accurate time-keepers, more convenient, and precise angle-measuring instruments for sea use, and more correct tables of the places and motions of the heavenly bodies, and especially of the moon.

The mode of determining the longitude by lunar distances had for a long time been advocated by the most eminent astronomers. The difficulties were the want of good instruments available at sea, and of tables of the moon, to obviate the necessity of mariners having to compute her place at each observation—a difficulty that put an almost insuperable bar to its adoption. The invention of Hadley supplied the first want admirably; and Maskelyne soon after met the other by inducing the Board of Longitude to publish annually a nautical ephemeris, containing the places of the sun and moon for every twelve hours, and the distance of the latter from the principal fixed stars. Hadley's quadrant, the ancestor of our present beautiful sextant, is an instrument with a graduated are, as in the old quadrants, but which by means of one fixed and one movable mirror enables the observer to reflect the images of two stars or objects, so as to appear in coincidence. The movable mirror is attached to an arm that traverses the graduated arc, the amount of movement necessary to bring the object into coincidence measures their angular separation, and the angular distance between the moon and the sun or a fixed star, when corrected by a careful calculation, gave, by the help of the lunar tables, the mean time at Greenwich; and this, compared with the local time at the place, at once furnished the desired longitude.

In the year 1714 the British Parliament passed an act holding out a great recompense to those who should contribute to the discovery of the longitude at sea—namely, £10,000 if the longitude were found within a degree, £15,000 if within forty minutes, and £20,000 if within half a degree. These magnificent rewards produced the desired effect. John Harrison, a man of humble origin but great genius, devoted a long and laborious life to the construction of clocks and watches for navigation. So numerous and important were his improvements that we may with justice consider him the inventor of the marine chronometer. It will, perhaps, give the best idea of his merits to say that he has done as much for this instrument as James Watt for the steam engine. In 1749 he obtained the Copley medal of the Royal Society, and in 1701 obtained from the Commissioners of the Board of Longitude a trial of his chronometer by a voyage to Jamaica. It was found at the end of sixty-one days that his chronometer gave the Longitude of Port Royal within five seconds of time; and on the return to England, after an absence of 101 days, the whole variation was only one minute five seconds. It was evident that page 10 the conditions of the Act of Parliament were satisfied, and Harrison received a payment, on account, of £5000; but the Commissioners decided that the remainder of the reward should not be paid till a second trial had taken place. This was executed in 1764, and crowned with complete success. It was decided unanimously by the Board of Longitude that the longitude of Barbadoes had been determined within the limits prescribed by the Act. £5000 was immediately granted to him, and £10,000 more when he had explained to Commissioners appointed for the purpose the details of his construction. This took place in 1765.

It is somewhat remarkable that an impression is still extant with some people that the reward above referred to has yet to be claimed. This country, like all others, has its inventors and men of one idea; and the amount of correspondence I have received during the last twelve or fifteen years from this class of individuals would scarcely be credited. Not a few among these correspondents are inventors of nautical instruments or new-methods of finding the longitude at sea, and until enlightened to the contrary have usually persevered in their frequently chimerical efforts, in the hopes of gaining the reward which was long ago given to Harrison.

The renowned name Herschell first appears upon the records of astronomical progress, in the period I am now referring to.

William Herschell (afterwards Sir William.) a self-taught but able astronomer, who had for a long time been engaged in constructing reflecting telescopes of great perfection and considerable dimensions, to enable him to pierce deeper into the mysteries of space, while engaged on the evening of the 13th March, 1781, in observing some stars in the constellation Gemini, noticed one that appeared different in character to the rest; the excellence of his telescope admitting of the use of eyepieces magnifying several hundred times, he found that the diameter of this object increased and assumed a well marked disc-like form, as he increased the power, while that of the fixed stars around it remained unaltered. At first he thought it was a comet, and communicated the discovery to Maskelyne and others, and in the course of two or three months it had been seen by nearly all the European astronomers; it was some time, however, before it could be decided whether it was a comet or planet, although its defined planetary appearance led most astronomers to believe it was the latter. Lexell was the first to show that its orbit about the sun was nearly circular, and it was therefore a planet, not a comet.

Herschell, the discoverer, named the planet Georgium Sidus, in honour of the King, George III., from whom he had received great and liberal encouragements; but this name did not meet with much favour among Continental astronomers. The famous Laplace suggested it should he called after Herschell himself, but the other planets having received mythological names, it was ultimately agreed to call it Uranus, which in the Greek fables is the name of the lather of Saturn, as the latter is of Jupiter.

In 1787, Herschell completed his great forty-feet reflector, the first of Brobdignagian telescopes (the last is in Melbourne;) with it he discovered two satellites of Uranus—Oberon and Titania. In 1789 he discovers two more satellites of Saturn's system, Mimas and Euceladus: and also suspects the existence of two more satellites in the Uranian page 11 system; and in 1798 he announces the discovery that the satellites of this planet revolve around him in an opposite direction to that of the satellites to other planets.

While on the subject of this planet, I might mention that it has generally been accepted that Uranus has eight satellites, six of which were found by Herschell, but Mr. Lassels states emphatically, after a long series of observations with his grand reflector, that there are only four or if there are more they remain yet to be discovered. The revolution of Uranus about the sun occupies about eighty-four of our years. Its diameter is about 33,000 miles, and its distance from the sun about nineteen times that of the earth from the sun.

We will now put on our astronomical "seven-league boots," and pass on to the year of grace 1870, and look back on the results of astronomical research during the last seventy years. It is unfortunate, however, that as I come to the thickest of my work, I become sensible that I am approaching nearer and nearer to the end of your patience.

Towards the beginning of this period the first of a series of discoveries was made by Piazzi and Olbers, which promises to be almost inexhaustible, so long as the perfection of telescopes increases in proportion. I refer to the planetoids. The first of these, Ceres, was discovered by Piazzi, in 1801; Pallas was discovered by Olbers in 1802; June in 1864, by Harding; Vesta in 1807, by Olbers; and so on, till these small members of the solar family number 109.

It had been long noted that there was a wide space between the orbits of Mars and Jupiter, and that there was a certain regular gradatory distance between the orbits of the other planets in some manner proportional to the distance from the sun. This regular order was, however, broken for want of a planet to fill this gap in the system, until the first planetoid Ceres was discovered.

The orbit of this body was found to very nearly coincide with the position required to complete the order of the distance from the sun—an order which is known as Bode's Law, by which it is assumed that the distances are as the numbers 4, 7, 10, 16, 28, 52, 100, 196, and 388; for the actual distances of the planets accord very closely to this; Neptune is, however, to some considerable extent an exception. The large numbers and minute dimensions of these bodies have led to the belief that they are fragments of some large planet which had in ages gone by been shattered to pieces. This supposition is rendered the more probable from the almost inexhaustible supply which seems to attend on each increase of the penetrating power of telescopes. The orbits of these little planets are so interlaced that we almost wonder that they do not stumble across one another in their circuits. If we were to represent the orbits, interlaced as they are in space, with rings, we could not lift one without lifting all the rest.

Numerous national observatories have been erected in this interval prominent among which are those at the Cape of Good Hope (1821,) at Parramatta, N.S.W. (1822, since dismantled,) Cambridge, in England (1823,) at Harvard College, U.S.A. (1840,) Washington (1842.) Williams-town, Victoria (1853,) Sydney (1858;) and so identified has astronomy become with national progress, that several large observatories have been page 12 established, and are maintained, by municipalities; instances of these are those of Manchester and Liverpool, while many of the highest class have been instituted in connection with colleges and universities for educational purposes.

The places and motions of the heavenly bodies are now known with great exactitude, and the charts, catalogues, and ephemerides of the moon, sun, stars, and planets are almost perfect; so precise, in fact, that the times of eclipses of the sun or moon, or of the forthcoming transits of Venus, can be predicted years beforehand, to commence at a certain time; and it shall be found that the times of prediction and occurrence tally within two seconds.

More giant telescopes had been constructed, both refractors and reflectors. Among the former I may instance the Dorpat and Pulkowa, one nine and a-half, the other sixteen, inches aperture; Cincinnati twelve, Harvard College fifteen, Greenwich twelve inches; and within the last year the largest refractor ever made has been completed. I refer to Mr. Newall's great twenty-five inch refractor, constructed by Cook and Son, of York.

Respecting the Pulkowa telescope, which was known as the "Great Pulkowa Refractor," Struve, the late Russian Imperial astronomer, said—"It might well be called the great refractor, as it broke the legs of himself and two of his assistants while they were erecting it."

There is one fact in connection with the splendid telescope at Harvard College which is worthy of notice—it was purchased with funds raised by subscription among the citizens of Boston, Salem, New Bedford, and Nantucket Mr. Bond says, in one of his reports—"It is worthy of note that no restriction or reservation was in any instance required (by the contributors) in regard to a right of visiting the observatory, or a control of its operations. This liberality on the part of the contributors has been productive of the most beneficial effects." The cost of the telescope and tower in which it is erected was about 15000, one thousand of which was contributed by President Quincy. The Cincinnati telescope is another instance of a similar kind, but in this case the contributions were not confined to money subscriptions; no inconsiderable portion of its cost was covered by donations in kind, of hams, preserves, and other produce, and a contribution of caps figures in the list.

Of large reflectors. Lord Posse's giant of Parsonstown stands first; then there are those of Mr. Lassels, Mr. Warren De La Rue, Nasmyth, and lastly, the great Melbourne reflector. Besides these, numberless magnificent telescopes of both kinds, but of smaller dimensions, are now nightly serutinising the skies in the hands, of both amateur observers and professional astronomers. In 1835 Sir John Herschell (the son of Sir William) commenced his famous series of observations of the southern nebulas and stars at the Cape of Good Hope, with his great reflector.—a task which occupied him five years. It is in following up and revising some of these observations that our great reflector is principally engaged. The results of Sir John Herschell's Cape observations are now published, and constiute the stadard authority on the nebulae of the southern hemisphere.

At Greenwich, Pond succeeded Maskelyne in 1812, and next came Airy, the present Astronomer-Royal, who was appointed in 1835.

The first measurements of the distances of the fixed stars were made in 1838 and 1839. Bessel determined that of 61 Cygni, and Henderson that of α Centauri. You will remember that the distance of the nearest of the fixed stars is immeasurably greater than that of the farthest planets, and that consequently the earth's diameter, the base line used in determining the distance of the planets, becomes uselessly small when we come to deal with the stars; instead of the earth's diameter, therefore the diameter of her orbit about the sun is taken as the base line, which gives us a length twice the sun's distance, or about 183,000,000 miles. The process of finding the distance, which is known in astronomical language as determination of annual parallax, depends upon ascertaining whether a star's true position remains the same when observed while the earth is in any certain part of her orbit, and again when in another part six months distant, in which case there will be, roughly speaking, 183,000,000 miles between the two points of observation; if the star's position appears unaltered, then it is said its parallax is insensible, and its distance so great as to be immeasurable. If ever so small a difference of position, however, is discernible, the distance can be deduced.

In the case of the two stars mentioned, the annual parallax of α Centauri was found to be 9-10ths of a second, which gives it a distance of 224,000 times the sun's distance; this star appears to be by far the nearest to us. The distance of 61 Cygni, the next nearest, is 366,000 times the sun's distance.

The fact that the solar system is actually moving through space, was by this time pretty well established; but there was considerable uncertainty as to the precise direction or rate of this movement. Sir John Herschell pointed out so long since as 1783 that the direction appeared to be towards a certain point in the constellation Hercules. Struve has now shown that this is actually the direction; and he says :—'The velocity of the motion is such that the sun, with the whole cortege of bodies depending on him, advances annually in the direction indicated at the rate of 154,000,000 miles per annum."

The greatest astronomical achievement during this period was, undoubtedly, the discovery of the planet Neptune—the last and most distant of the major planets now known.

The circumstances attending this discovery are, I have no doubt, well remembered by most of you. Two mathematicians were (independently and unknown to each other) engaged in investigating as to the cause of certain irregularities exhibited by Uranus in his orbital motion, which could only be accounted for by supposing the disturbing cause to be an unknown and large planet still more distant from the sun. Both these mathematicians (M. Le Verrier of Paris, and Mr. Adams of Cambridge) sent the results of their calculations to the principal European astronomers, indicating the position in the heavens in which the supposed planet might be found. A systematic search was commenced, and on September 23rd, 1845, M. Galle, an assistant in the Berlin Observatory, found the stranger close to the position indicated by Le Verrier and Adams.

This new planet has been called Neptune. The full significance of this discovery may not at first, perhaps, be so apparent to those unacquainted with the whole circumstances; but, when we come to consider page 14 that the actual position in space occupied by an unknown and invisible body, the only indication of whose existence was the disturbing effect on the motion of Uranus, was almost precisely pointed out by those mathematicians, after calculations involving the most difficult and complicated considerations, we shall be prepared to endorse the words of Mr. Hind who says :—

"A most brilliant discovery,—the grandest of which Astronomy can boast,—and one that is destined to be a perpetual record in the annals of science—an astonishing proof of the power of the human intellect."

In all other cases, the one who first sees has reaped the whole honour due; but here the mere finding falls far into the shade when compared with the intellectual skill that enabled Adams and Verrier to tell astronomers where to find it.

The dimensions of Neptune are not yet known with certainty, on account of his immense distance from us; his diameter is, however, estimated at about 37,000 miles. He occupies 164.6 years in his revolution about the sun, at a distance of about 274G millions of miles.

The origin and nature of meteors have long presented a fertile field for speculation and theorising. Within the last few years, however, our knowledge of these minute denizens of space has been very considerably increased, especially since the celebrated meteor shower of November, 1866. Long prior to this, however, it had been remarked that on certain occasions meteors had been seen in unusually large numbers, and Humboldt describes a grand shower of them he witnessed in 1799. Oldsted, in comparing this with former occurrences, adopted the theory that these meteoric storms are due to the progress of the earth through a mass of these atoms, which appear to be congregated into a ring which intersects the earth's orbit. Professor Newton, of America, and others, have still further investigated the matter, and their conclusions tend to establish Oldsted's theory; and it is now considered that ordinary shooting stars or meteors are minute particles of cosmical matter which revolve about the sun in an orbit or ring, like the planets; that they are chiefly congregated into two rings, one intersecting the plane of the earth's orbit in August with an inclination of 79°; the other in November, with an inclination of 17°; and also, that in some portions of these rings the meteors are more densely crowded than at others. They are called the November and August rings, from the fact that they intersect that part of the ecliptic occupied by the earth in those months. In these months, therefore, meteors are usually for more numerous than at others; but it is only when the crowded portions of the rings cross the earth's path at those months that we have those grand displays called meteor showers, the last of which was witnessed in Northern Europe in November, 1866. Professor Newton shows that so far as the November ring is concerned, these showers occur every thirty-three years, although the grandest of them will come only at periods of about 133 years.

The motion of these minute bodies is contrary to that of the earth; we therefore meet them "full tilt." They all appear to come from one point in the heavens within the constellation Leo. Mr. Alexander Herschell, one of our highest authorities on this class of astronomy, states, as one of page 15 the results of his observations of the August fall of 1866, that not one in twenty of these bodies exceeds a pound in weight, most are very much smaller, and some cannot exceed a few grains. It appears pretty certain that these bodies become visible to us while yet at a distance of from seventy to fifty miles; and why such small masses at such a distance can produce the brilliant light with which they become visible is accounted for by the immense velocity (thirty miles a second) with winch they approach us, becoming converted into heat immediately they enter our atmosphere.

Photography has of late years come into extensive use in astronomy. The first application of it in this direction is due to Dr. Draper, of New York, who obtained photographs of the moon in 1840. Mr. Bond, of Harvard College, seems to be the next to use it, in 18.30, since which it has come into very general use; and in the hands of Mr. Warren De La Rue, Rutherford, Draper, and others, has now reached a very high state of perfection. The moon has naturally received the greatest attention from photographers, and some very magnificent pictures have been published from time to time. Photographs of the brighter planets, double stars, and star clusters, have also been successfully obtained; and at the Kew Observatory, pictures of the sun's surface are taken every clear day. In total eclipses of the sun, also, photography has proved eminently useful in securing for all time some of the strange and evanescent appearances witnessed on such occasions.

The application of the electric telegraph to observational astronomy belongs to the late portion of the period I am dealing with. It was first used by Dr. Locke, in America, in 1848, and the galvanic current has now become one of the essentials to a first-class observatory. The observer has no longer to listen painfully to the ticks of his clock or chronometer, as he watches the progress of the stars across his telescope, or the moments at which various phenomena occur: his undivided attention can now be devoted to the ocular part of his task. With a telegraphic key in his hand, he can at a touch send a signal to the chronograph, which records the desired instant with greater precision than could otherwise be obtained. The extended application of chronography by means of the galvanic current has opened up new modes for the solution of many very important astronomical problems.

So far as our knowledge of the condition and constitution of the heavenly bodies is concerned, there is no modern accessory in astronomy that has done so much as spectrum analysis. By its aid the constitution of the visible portion of the sun has been revealed to us with certainty as consisting of incandescent vapours, among which are the vapours of many of the substances and gases we know on the earth's surface, notably sodium, iron, nickel, magnesium, barium, calceum, and hydrogen. On the occasion of the last total eclipses visible in India and America, the spectroscope unravelled the mystery of those strange red prominences' which are seen to jut out from the sun's edge during the moments of totality. It showed them to be incandescent hydrogen gas—jets of flame projecting for tens of thousands—nay, almost a hundred thousand miles into space, of such gigantic dimensions that they would embrace dozens of such puny globes as ours in their fiery arms, like motes in a candle.

By an ingenious arrangement of spectroscope with the telescope, these red flames can now be witnessed at almost any time without an eclipse, with which they have nothing whatever to do, but which become the more readily visible on these occasions on account of the complete obscuration of the sun's direct glare. The spectroscope has also shown that the fixed stars are in all respects suns. Lines indicating the presence of many of the substances known in the earth, and in a state of vapour in the sun, are easily recognised on numbers of them. Hydrogen appears to be a common constituent among the stars, and in some the hydrogen spectrum becomes the most prominent. Some years ago a star, T. Coronœ, was observed to suddenly grow brighter: before the change it was so small as to be invisible to the unaided eye, but in a few days it became as brilliant as the brightest stars. The spectroscope told us that the sudden increased brilliancy was due to an outburst of hydrogen, far outvieing in magnitude the most stupendous flames that are ejected into space by our sun. This occurrence, with respect to T. Coronœ, has given rise to speculations, as our sun is continually vomiting forth these monster flames, whether there may not be occasions when some such terrific outbursts take place on it: but cases like T. Coranœ: are the very rare exception, and not the rule; and few instances of such rapid variability in the brightness of stars are on record.

Before concluding. I would like to refer to one item, and that not the least in the past seventy years of Astronomy: the establishment of an observatory in Victoria. Like Greenwich, it was established to facilitate navigation; that was, and still is, its prime object. Its beginnings took place in 1853, when I was invited to form and conduct an observatory at Williamstown, for the purpose of obtaining and maintaining accurate local time, and of giving public time signals to enable masters of ships to obtain errors and rates of their chronometers. By this time chronometers had been brought to such a state of perfection, that the method of obtaining longitude at sea, by the transport of Greenwich time, was almost exclusively practised Chronometers, however, of the best construction are but machines liable to break down, stop, or go wrong: the Lunar distance method therefore affords a valuable stand-by in case of any such contingencies, and no good seaman is master of his art, or a safe navigator, unless he is up to "Lunars." That they are not always so, is unfortunately a fact, for which, perhaps, the excellence of chronometers is to blame. I recollect, some years ago, an excellent old salt whom I had known for several years, bringing his chronometers to the observatory on his arrival in port; one of them had gone wrong, and the other was not quite trustworthy: he had been in a quandary. I said to him, I suppose you made yourself safe by a few "Lunars." "Lunars !" he said, "No, sir. I only took two 'Lunars' since I was master of a ship: once I was about the meridian of Ascension, and my 'Lunar' put me about the middle of the desert of Sahara, and next I was near St. Paul's Island, and it put me pretty nigh-the meridian of Cape Leuwin, so I thought 'Lunars' were not much account, and never took any since. I knocked along this time with the log and my lame chronometer, and I guess I'm in port safe enough now."

Like Flamsteed. I commenced with indifferent instruments. For about twelve months a sextant and chronometer were the only ones of which our page 17 National Observatory could boast. Through the liberal support that has ever been accorded to science by our Governments and Legislature however, the Observatory in the course of a few years grew out of its swaddling clothes, and became a well-equipped establishment. After nine or ten years it was removed from Williamstown to its present site.

Our Observatory was one of those which undertook a series of observations for the determination of the parallax of Mars at its opposition in 1862 for a new measurement of the sun's distance; and the Williams-town series has been acknowledged to be the most successful obtained on that occasion. The sun's parallax derived from the combination of our with the Greenwich observations was 8″ 93, while that from the Pulkowa and Cape of Good Hope was 8″.96—a very close coincidence. The most noteworthy fact connected with these results is that they agree so closely with the parallax assigned by Le Verrier and Foncault on other grounds.

Light had hitherto been assumed to travel at the rate of 192,000 miles a second. Foncault's beautiful experiments showed its speed to be only 185,170 miles a second. From eclipses and other phenomena we know light takes eight minutes eighteen seconds to come from the sun to us. A simple calculation, then, gives 92,000,000 as the distance and 8″.86 as the parallax. Le Verrier assigned a parallax of 8″.95, which resulted from his investigations of the perturbations of some of the planets. The parallax resulting from the transit of Venus in 1769 was 8″.58, equal to a distance of a little over 9,5,000,000 mile; that from the Mars observations gave a distance of 91,500,000 miles—a difference of three and a half millions miles. Were it not for the coincidence of the Mars results with those deduced by Foncault and Le Verrier, there might be some reason to doubt the results. Lately, however the Venus observations of 1709 have been re-discussed by Mr. Stone, of Greenwich, who finds that by applying some necessary corrections to the observations, which certain appearances on a late transit of Mercury had indicated to be requisite, the results coincide almost exactly with those obtained in 1862.

The astronomical work that has engaged our Observatory is, as at Greenwich, the Cape, and many other national observatories, confined almost exclusively to the utilitarian class. The maintenance of correct local time, and giving public time signals, to enable mariners to rate their chronometers correctly and with facility; the determination and correction of the places of the principal fixed stars; and a complete survey of certain portions of the southern heavens, for the formation of accurate star catalogues and charts of the southern hemisphere (a work that has been undertaken by this in co-operation with two other British observatories,) constitute the principal routine work. Since the erection of our great telescope, observation of and depicting the southern nebulæ have been added to this. The Melbourne star catalogues of the southern heavens are now admitted to be the most valuable extant, and it appears they have become of special importance with reference to the forthcoming transit of Venus; so that whatever our colony may do on that occasion, she has already contributed a very honourable share in these star catalogues alone.

It is a very popular belief that practical Astronomy consists of star-gazing, looking at the moon, or searching for some unknown stars, planets page 18 or comets; and young ladies who come to the Observatory on fine, pleasant nights, to have a keek at the moon, say, "How delightful ! What a beautiful, romantic occupation !" and so it is for an hour or two, if it is not too cold; but long, weary hours of monotonous watching and recording at night, with the long, dreary calculations of to-morrow looming ahead, soon strip the delightful romance from the calling of the practical astronomer, and bring it down to the level of other bread and butter getting occupations. There is no such thing as star-gazing (as that term is generally understood) in a regular observatory;—every observation is for a particular purpose, and set out beforehand. The more popular branches of Astronomy, such as comet and planet seeking, investigations of the moon's surface, delineations of the features of the nearer planets, and such like, are, by general consent, left to amateurs and private individuals, many of whom are in possession of magnificent telescopes, well fitted for the work, and who yearly contribute very largely to our knowledge on these subjects. National institutions, however, have to grind away at the harder stuff, which does not present enough excitement, or requires too much application, for those who follow up astronomy for amusement. This is how it is we nearly always hear of comets, and even planetoids, being discovered by such observers, and not by those who have to do the less genial but more utilitarian work.

The most important work in astronomy, as in other sciences, is that which appeals least to the uninitiated and the general public; hence the cui bono question so frequently advanced with respect to observatories and scientific institutions; and I recollect an occasion when an astronomer engaged upon some work of the highest, importance and real commercial value, was severely blamed in public print for allowing a zealous young amateur astronomer to discover a comet! Greenwich Observatory has been established nearly two centuries, yet neither a comet, planet or planetoid has ever been first discovered there. For all this, the basis of modern astronomy is admitted by all astronomers to rest on the Greenwich Observatory.

As an example of the value—the commercial value—of the simple maintenance of true time, one of the first objects of a national observatory, I may mention Mr. Warren De la Rue, the celebrated paper manufacturer, states that "he estimates the annual saving to his firm, by having exact time, and enforcing strict attendance on his work-people, at £300 per annum (besides some saving of gas and coals, not taken into account,) which is an amount that would otherwise be entirely lost; and of this he is able to make a return to his work-people in the way of additional privileges as respects holidays."