17( 



WELLS'S NATURAL PHILOSOPHT. 



tJpon what 

 p inciple are 

 fl s enabled to 

 w Ik upon the 

 ceiling, etc.? 



leet. 



Explain the 

 principle and 

 construction of 

 the exhausting 

 Byrin:^e and air- 

 pump ? 



Fig. 1G5. 



by it. The effect of the sucker arises from the exclusion of the air between 

 the leather and the suri'ace of the stone. The weight of the atmosphere 

 presses their surfaces together with a force amounting to 15 pounds on every 

 square inch of the surface of contact. If the sucker could act with full 

 eliect, a disc an inch square would support a weight of 15 pounds; two 

 square inches, 30 pounds, etc. The practical effect, however, of the sucker 

 is much less. 



388. The power of flies and other small insects to walk on 

 ceilings, and surfoces presented downward, or upon smooth 

 panes of glass, in opposition to the gravity of their bodies, is 



gL-ucrally refcred to a sucker-like action of the p;dms of their 

 Kecent investigations have, however, proved, that the effect is rather 

 due to the mechanical action of certain minute hairs growing upon the feet, 

 which are tubular and excrete a sticky liquid. 



389. For the purpose of exhibiting the effects produced by 

 the atmosphere in different conditions, and for various practi- 

 cal purposes, instruments have been contrived by which air 

 may be removed from the interior of a vessel, or condensed 

 into a small space to any extent, within certain limits. Tho 



first of these requirements may be obtained by the use of the instruments 

 known as the exhausting springe and the air-pump. 



Tlie exhausting syringe consists of a hollow cylinder, generally 

 of metal, B C, Fig. 165, very truly and smoothly bored upon the 

 inside, and having a piston moving in it air-tight. This cylinder 

 communicates by a screw and pipe at the bottom, with any ves- 

 sel, generally called a receiver, from which it is desirable to with- 

 draw the air. The piston has a valve at E, opening upward, 

 and at the bottom of the cylinder another valve precisely similar 

 is placed, which also opens upward, shown at A. Suppose 

 now the piston to be at the bottom of the cylinder and the re- 

 ceiver to be in proper connection — upon raising the piston by 

 the handle, D, a vacuum is made in the cylinder ; immediately 

 the air in the receiver expands, passes through the valve A at 

 the bottom of the cylinder, and fills its interior ; upon depressing 

 the piston, the valve E opening upward permits the air to pass 

 through, and the valve A at the bottom of the cylinder closing, 

 prevents it from passing back into the receiver. Upon again 

 raising the piston, a further portion of air expanding from the 

 receiver, enters the interior of the syringe, and upon depressing the piston, 

 passes out through its valve. It is evident that this operation may be con- 

 tinued as long as the air within the receiver has elasticity sufiBcicnt to force 

 open the valves. 



The process of removing air from a vessel, or receiver, by means of the ex- 

 hausting syringe is slov.' and tedious, and more powerful instruments, known 

 as air-pumps, are generally employed for this purpose. The modern form 

 of constructing the air-pump is represented by Fig. 166. The principle of ita 



