292 SURVEY OF THE COAST 



times, and one of the niifldle ones prismatic. There is a 

 lengthening tube of alioiit five inches in length to the object 

 end, to keej) off the side Ught and shelter the object glass. 

 It balances at the same time the opposite end of the teles- 

 cope. Then equilibrium is estal>lished in all positions of the 

 telescope without clamping, which always more or less affects 

 the accuracy of its position. 



In the focus of the telescope there are three fixed verti- 

 cal wiles, and one horizontal one moveable i)y a micrometer 

 arrangement similar to that of the reading microscopes, by 

 which sjnall differences of level can be measured, as a very 

 fine large spiiit level can be hung lengthways to the telescope 

 by two a(ljustai)le steel pins, on the side of the tube not seen 

 in the figure. The vertical wiies are of course adjustaltle 

 like those of any transit instrument, by two opposite screws 

 on the sides, as d'. 



The middle piece of the transit is a zone of a sphere of 

 five inches in diameter, to which the two parts of the teles- 

 cope are screwed in diametrically o()posite directions, and at 

 right angles to these the two truncated cones, forming the 

 axis. Their base is a circle drawn on the cord of about i20° 

 of the central sphere. It diminishes to three-fourihs of an 

 inch at the other end, to receive the bell metal axis piece 

 of half an inch in diameter. This shape presents a great 

 strength of support against the sinking of the telescope by its 

 own weight, wliile the central sphere is much lighter than 

 the square formerly used, without any loss of strength. 



This axis is perforated on one side to admit the liglit of 

 a lantern placed on a piece projecting from the top of the 

 column. A plane white glass is placed at the end of the 

 axis, and in the si)heric centre piece a plate, at an angle of 

 45°, perforated for the passage of the rays to the telescope. 

 It is covered with gold leaf left unpolished, to prevent the 

 glare of the reflected light on the wires. It admits of a small 

 adjustment by the fastening screw t in the middle of the cen- 

 tral sphere. 



The lantern beaver is very light, slides from outside on 



