100 



BENJ. PIKK'b, JR., DESCRIPTIVE CATALOGUE. 



and to prevent the wind giving a tremulous motion to its 

 surface, a piece of thin gauze, talc, or plate-glass, whose 

 surfaces are perfectly plane and parallel, may be placed 

 over it, when used for observation. But the most accurate 

 kind of artificial horizon is that in which fluid quicksilver 

 forms the reflecting surface, the containing vessel being 

 placed on a solid basis, and protected from the influence of 

 the wind. The adjoining figure (No. 112) represents an 

 instrument of this kind. The mercury 

 is contained in an oblong wooden trough, 

 placed under the roof A, in which are 

 fixed two plates of glass, whose surfaces 

 are plane and parallel to each other. 

 This roof effectually screens the surface 

 of the metal from being agitated by the wind, and when it 

 has its position reversed at a second observation, any error 

 occasioned by undue refraction at either plate of glass will 

 be corrected. Price, $20.00 and $25.00. 



Fig. 112. 



Fig. 113. 



Another and more portable contrivance 

 for an artificial horizon, is represented in 

 the annexed figures, which consists of a 

 circular plate of black glass, about two 

 inches diameter, mounted on a brass 

 stand, half an inch deep, with three foot 

 screws, a b c, to set the plane horizontal ; 

 the horizontally being determined thus 

 by the aid of a short spirit-level, d, hav- 

 ing under the tube a face ground plane, 

 on which it lies in contact with the re- 

 flecting surface ; place the level on the 

 glass, in a direction parallel to the line 

 joining two of the three foot-screws, as 

 a and b, then move one of these screws 

 till the bubble remains in the middle of 

 the tube, in both the reversed positions 

 of the level, and the plate will be hori- 

 zontal in that direction ; then place the 

 level at right angles to its former posi- 

 tion, and turn the third foot-screw back 

 or forwards till the bubble again settles 

 in the middle of its tube, the former 



