Spirit Level. 321 



It will be seen by the foregoing table, that there are gases 

 much lighter than the atmosphere ; accordingly, if a large quan- 

 tity of one of these fluids were confined by a thin covering, as oiled 

 silk, it would rise in the atmosphere as a cork does in water until 

 it reached a region of the atmosphere of the same specific gravity 

 with itself. It is upon this principle that balloons are construct- 

 ed. They were at first filled with air rarefied by heat, and a fire 

 was supported in a car placed under the balloon for this purpose. 

 Hydrogen was afterward discovered, and proved to have a specific 

 gravity only T \ of that of common atmospheric air. This gas, 

 is the lightest of known fluids, and will consequently require a 

 less bulk for a given buoyancy than any other. It is universal- 

 ly employed in the construction of balloons. 



Spirit Level. 



436. This instrument consists of a tube AB, nearly filled Fig.213. 

 with alcohol or ether, the remaining space CD being occupied 

 by a bubble of air. Being attached to the vertical or horizon- 

 tal circle of a theodolite or other instrument, it serves to deter- 

 mine the horizontal position of a line or plane, since if either end 

 of the tube is higher than the other, the bubble of air, from its 

 specific lightness, will indicate it by tending to the higher part. 

 It is usual, where great exactness is required, to make the tube 

 slightly curved, the curvature being circular ; the bubble will 

 then move more readily, settle itself with more certainty, and 

 describe equal spaces by equal changes of inclination. A good 

 spirit level will exhibit a movement of more than half an inch for 

 each minute of inclination, and the bubble will sensibly alter its 

 position by a change of five seconds in the inclination. In such 

 a tube, the radius of curvature will be about 150 feet. But lev- 

 els have been rendered still more delicate. Lalande speaks of 

 one, filled with ether, the bubble of which passed over fourteen 

 inches by equal spaces of one tenth of an inch for every second. 

 The radius of curvature was consequently 1719 feet or nearly 

 one third of a mile.t 



t The number of seconds in a circle is 



360 X 60 X 60 = 1296000, 

 Mech. 41 



