SURVEYING IXSTIIUMENTS. 99 



the latter is purposely prevented from moving. The interior 

 axis is furnished with a milled-head, c, and the exterior with 

 a milled-head, B, also ; so that when the head, c, is held 

 and the other turned, the circle may be moved independent- 

 ly of the smaller axis ; and when the outer one, B, is held, 

 and the inner one, c, turned, the smaller axis may be turned 

 independently of the circle. Attached to the end of the 

 smaller axis is a sort of universal joint, D, capable of being 

 fixed in different positions. The crystal to be examined is 

 attached to the joint at F, by a little soft wax, and placed 

 so that its edge shall be parallel to the axis of motion ; 

 which adjustment is obtained by placing it so that the image 

 of some horizontal object, as the bar of a window, succes- 

 sively reflected from the two faces of the crystal, coincides 

 with another horizontal line seen by direct vision. When 

 this adjustment has been made, the instrument is turned till 

 the horizontal object is seen reflected from one of the faces. 

 The smaller axis is then held fast, and the other turned till 

 the index of the vernier points to the zero of the graduated 

 limb. The circle is then turned round, along with the 

 smaller axis, till the same object is seen in the same position, 

 by reflection from the other face of the crystal ; when the 

 arc passed through by the circle is obviously the supplement 

 of the angle formed by the two faces of the crystal. In 

 order, however, to avoid calculation, the supplements of the 

 angles are marked on the limb, so that the angle to be 

 measured is read off immediately. Price, $20.00. 



The Artificial Horizon. When the altitude of a celestial 

 object is to be taken at sea, the observer has the natural, 

 or sea horizon, as a line of departure ; but on shore, he is 

 obliged to have recourse to an artificial one, to which his 

 observations may be referred : this consists of a reflecting 

 plane, parallel to the natural horizon, on which the rays of 

 the sun or other objects falling, are reflected back to an eye 

 placed in a proper position to receive them ; the angle 

 between the real object and its reflected image being then 

 measured with the sextant, is double the altitude of the 

 object above the horizontal plane. 



Various natural, as well as artificial, reflecting surfaces 

 have been made by mechanical arrangements, to afford the 

 means of obtaining double angles ; such as pouring water, 

 oil, treacle, or other fluid substances into a shallow vessel ; 



