56 BENJ. PIKE 8, JR., DESCRIPTIVE CATALOGUE. 



sists of a brass slip, having a narrow vertical slit ; the other, 

 o, which is turned to the object, is a similar slip, having an 

 oblong aperture containing a fine thread, passing along the 

 middle of the open space in a vertical direction. Two ver- 

 tical lines are also marked on the inside of the box, which 

 are prolongations of the slit in the sight for the eye, and of 

 the thread in that for the object. These lines are intended 

 as indexes for the measurement of the angular distance in 

 azimuth of an object viewed through the two sights, from 

 the place of the magnetic meridian, as shown by that por- 

 tion of the graduated edge of the card which coincides with 

 the line with which it is compared. 



Price, 88.00 to $20.00. 



The Prismatic Compass. (Fig. 89, as below.) The use 

 of this little instrument is to measure horizontal angles only, 

 and from its portability it is particularly adapted for military 

 surveying, or where but little more than a sketch map of the 

 country is required. It is also very useful in filling in the 

 detail of a map, where all the principal points have been 

 correctly fixed by means of the theodolite. It may likewise 

 be used for determining approximately the direction of the 

 true meridian, the variation being determined by comparing 

 the observed azimuth of a celestial object, with its true 

 azimuth deduced from an observation made for the purpose. 



Fig. 89. 



