Description of a Theodolite.

For accurate surveying an instrument of great precision, called a theodolite, is used.

Though to the uninitiated this instrument appears extremely complicated, its manipulation is simple. The theodolite is used only for measuring angles, both vertical and horizontal, and for placing marks, or stations, in a straight line in any desired direction. It is not used for the purpose of making calculations, as it is popularly supposed.

There is a story of a bush surveyor buying a dead pig for camp meat from a Maori. The price agreed upon was 3d. a pound. The Maori was told to hang the pig on a tree. The surveyor had a look at it through the theodolite. He estimated the pig would weigh 100lbs., so he told the Maori the instrument gave the weight as 60lbs., which at 3d. a pound, the instrument calculated as 12s. 6d. The Maori accepted this sum with bad grace, but returned the next day with a battered ready reckoner wherein 60lbs. at 3d. a pound was correctly shewn as 15s. The surveyor asked to have a look at the book, and then informed the Maori that the book was no good as it was last year's and out of date, so the Maori was quite satisfied, being convinced of the calculating powers of the theodolite.

The instrument is mounted on three strong legs to give rigidity, and is set up directly over a mark, or station, by a “plumbob” hanging from the instrument. The machine is levelled up true by thumb screws operating a spirit bubble, similar to that on a carpenter's level. For horizontal angles two flat circular plates, about six inches in diameter and half an inch thick, move one above the other. The edge of the lower plate is graduated with great precision shewing degrees and half degrees, and on the upper plate is a scale shewing further graduations in minutes. The moving of one plate on the other permits of any angle through which the plates have moved being read. A magnifying glass is used for accurate reading. For vertical angles two similar plates with the graduated markings are mounted vertically on the machine. The complete circle is divided into 360 degrees, each further divided by the upper scale into 60 minutes, and further divided into 60 seconds. The circle is thus divided into no less than 1,296,000 parts. In practice it is only possible to read to one- page 12 third of a minute with the small theodolite in general use, and this gives a range of 64,800 different angles.

Mounted on the horizontal axis of the instrument is a small telescope, in the eye-piece of which fine cross lines at right angles fix the central point in the line of sight.