How Levels are Taken.

Intimately associated with engineering surveying is the necessity for accurate levels to determine heights, fix gradients and earthwork quantities. This work is carried out with an instrument known as the dumpy level. The instrument comprises a telescope mounted on three rigid legs and fitted with a very sensitive spirit bubble parallel to the telescope. The telescope and levelling bubble revolve freely, horizontally. The eye-piece of the telescope is fitted with two fine vertical cross lines and one horizontal crossing at right angles. The instrument is fitted with thumb screws on the top of the leg mounting to bring the telescope into a horizontal position in any direction. The telescope has considerable magnification for distances up to 300 yards and onehundredth of a foot can be observed at 100 yards distance. A graduated staff about 14 feet long and three inches wide, marked in feet, tenths and hundredths of feet, is employed in observing the heights of various points in relation to the line of sight through the level telescope.

In figure No. 16 is shown the method of taking a series of levels with the dumpy level. In the first series of readings A B C D and E it will be seen the readings are 12, 3, 11, 7 and 5 respectively. That is to say B is 9 feet higher than A, C is 1 foot higher than A or 6 feet lower than E and so on. The instrument is then moved from station 1 to station 2 and a back sight taken on the last reading from station 1 at E. This connects the second series with the first series—a procedure repeated again and again until the area to be covered is completed. As a check, levels are taken on the completion of the work back to the starting point as at A, and if the work has been well done the same level is arrived at, or at least within allowable limits. The levels of each reading are then tabulated, the original level being related to mean sea-level by connection to fixed monuments known as bench marks (much the same as surveying trigs), and available throughout the country. When thus tabulated the varying heights of each point of observation above mean sea-level are shown, and any one point can be compared with any other point as to their relative heights.