SURVEYING INSTRUMENTS. 87 



tLe diameter is increased, the other parts must be made 

 proportionably large and strong, and the instrument becomes 

 too weighty and cumbersome to be easily carried from station 

 to station. The object of increasing the dimensions, is to 

 enable the instrument to furnish more accurate results, by ap- 

 plying a telescope of greater power, and by a more minute 

 subdivision of the graduated arcs. With the increase of 

 size, a small variation takes place in the construction, prin- 

 cipally consisting in the addition of a second telescope, and 

 in the manner of attaching the supports, K and L, to the 

 horizontal limb, to afford the means of adjusting the hori- 

 zontal axis, and of course, making the telescope and vertical 

 arc move in a vertical plane. In the smaller instruments 

 this is done by construction, but in the larger ones, the sup- 

 ports, K and L, are attached to a stout frame, which also 

 carries the compass-box, instead of being fixed, as repre- 

 sented in our figure, to the upper horizontal -plate. The 

 frame is attached to the limb by three capstan-headed 

 screws forming an equilateral triangle, two of them lying 

 parallel to the horizontal axis, and the third in the direction 

 of the telescope ; the adjustment is made by means of these 

 screws. To prove its accuracy, set up the theodolite in such 

 a situation that some conspicuous point of an elevated 

 building may be seen through the telescope, both directly 

 and by reflection, from a basin of water, or, what is better, 

 of oil or quicksilver. Let the instrument be very correctly 

 levelled, and if, when a vertical motion is given to the tele- 

 scope, the cross-wires do not cut the object seen, both 

 directly and by reflection, it is a proof that the axis is not 

 horizontal ; and its correction is effected by giving motion to 

 the screws above spoken of, which are at right angles to the 

 telescope, or in the direction of the horizontal axis. The 

 third screw, or that which is under the telescope, serves for 

 adjusting the zero of altitude, or vernier of the vertical arc. 

 A second telescope is sometimes attached to the instru- 

 ment beneath the horizontal limb ; it admits of being moved, 

 both in a vertical and horizontal plane, and has a tangent- 

 screw attached for slow motion ; its use is to detect any 

 accidental derangement that may occur to the instrument 

 whilst observing, which may be done by it in the following 

 manner. After levelling the instrument, bisect some very 

 remote object with the cross-wires of this second telescope, 

 and clamp it firm ; if the instrument is steady, the bisection 



