88 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



will remain permanent whilst any number of angles are 

 measured, and by examining the bisection from time to time, 

 during the operation at the place where the instrument is 

 set up, any error arising from this cause may be detected 

 and rectified. 



Price, 5 inch, brass arches, $100.00 to $120.00 

 " 5 " silver " $125.00 

 " 6 " " " $150.00 

 " Y " " " $250.00 to $300.00. 



The Spirit Level (Fig. 100) con- 

 sists of a glass tube, nearly filled 

 with a liquid, spirits of wine being 

 now used, on account of its mobility, and not being liable 

 to freeze, the bubble of which, when the tube is placed 

 horizontally, would rest indifferently in any part, if the tube 

 could be made mathematically straight ; but that being im- 

 possible to execute, and every tube having some slight 

 curvature, if the convex side be placed upwards, the bubble 

 will occupy the higher part ; such a tube firmly fastened on 

 a straight bar, and marked at two points distant by the 

 length of the bubble, if so placed that the bubble shall 

 occupy this interval, has one definite relation to the horizon ; 

 for were it ever so little moved one way or other, the bubble 

 would shift its place, and run towards the elevated side. 



The accuracy of the indications of the level depends in a 

 considerable degree on the regularity of the interior surface 

 of the tube. They are commonly made of glass tubes, in 

 the same state as they are obtained from the glass-house ; 

 but when very great accuracy is required, the inside is 

 ground, to give them a regular spherical curvature. The 

 larger the bubble, the more freely it moves. The spirit 

 level is extensively used in instruments for surveying, and 

 for astronomical and geological purposes ; the glass tube 

 being inclosed in a brass case, which is cut out on the upper 

 side, so that the bubble may be seen. 



The Pocket Spirit Level. (Fig. 101, next page.) This cut 

 represents a brass pocket spirit level ; they are made from 

 three to twelve inches long, are mounted on a stout brass 

 plate, having the bottom ground true, and supported by a 

 small pillar at each end, on the upper part of which are 

 two nuts, between which the level is supported by projec- 



