SURVEYING INSTRUMENTS. 95 



parallel plate-screws : the capstan-screws, e, f, have brass 

 covers to defend them from injury or accidental disturbance, 

 but admit their adjustment when necessary. 



The spirit level itself has no adjustment, being firmly 

 fixed in its cell by the maker, and therefore the line of col- 

 limation must be adjusted to it, by means of two screws, 

 near the eye-end of the telescope ; the manner of doing this 

 is as follows : Set up the instrument on some tolerably level 

 spot of ground, and, after levelling the telescope by the 

 parallel plate-screws, direct it to a staff held by an assistant 

 at some distance (from ten to twenty chains) ; direct him by 

 signals to raise or depress the vane, until its wire coincides 

 with the horizontal wire of the telescope (or central division 

 of the micrometer scale): now measure the height of the 

 centre of the telescope above the ground, and also note the 

 height of the vane on the staff; let, for example, the former 

 be four feet and the latter six, their difference shows that 

 the ground over which the instrument stood is two feet 

 higher than where the staff is placed. Next make the 

 instrument and staff change places, and observe in the same 

 manner as before, and if it gives the same difference of level, 

 the instrument is correct ; if otherwise, take half the differ- 

 ence between the results, and elevate or depress the vane 

 that quantity, according as the last observation gives a 

 greater or less difference than the first. Again, direct the 

 telescope to the staff, and make the coincidence of the hori- 

 zontal wire and that on the vane perfect, by turning the 

 collimation screws. Price, $100. 



Levelling Staves. (Fig. 107.) A mahogany Fi - 107 - 

 rod, about 12 feet long, and two inches wide, 

 is divided into feet and hundredths. A target 

 of brass, about 8 inches square, or round, 

 slides on the rod, by means of a brass box, on 

 the back, having a spring to give ease and 

 regularity to the movement ; on the face of the 

 target is an aperture, over the divisions of the 

 rod, having a vernier, which reads to thousands 

 of a foot. The face is varnished in sectors of 

 different colors, as white and black, affording 

 a very distinct dividing line to the observer. 

 A stout cord is fastened to the upper part of 

 this box, on one side, and is carried around the whole length 



