of an Altitude may he measured by one Observation. 341 



with an additional level, and fixed in a vertical position, would 

 serve not only to take an altitude, but also to obtain, after 

 the completion of one single observation, any multiple of the 

 angle. With the aid of the front view of the circle, fig. 1., 

 and the horizontal section through its axis (fig. 2.}, its con- 

 struction, and the method of using it, may be briefly described. 



Fiff. 1. 



Fig. 2. 



53 



W is an eleven-inch wheel, furnished with an axis D, which 

 projects about three inches beyond each surface of the wheel 

 C, a ten-inch graduated circle, of wliich the hollow axis E 

 moves about the front part of the axis of the wheel. H, H are 

 two opposite verniers, fixed on the wheel, by which the divi- 

 sions of the circle are read off" to 10". P is an immoveable 

 (circular) jjlate, twelve inches in diameter, having a hollow axis 

 F, in which the back part of the axis of the wheel is fitted 

 and revolves. The two-foot achromatic telescope T, carrying 

 a spirit-level U, rests with its cylindrical rings within two Ys, 



