PNEUMATICS. 157 



strong enough to overcome the adhesion of the 

 bladder forming the lower valve ; which, conse- 

 quently, will remain shut, and the exhaustion 

 cannot proceed. Or, before this period, it may 

 happen, that the air between the valves when the 

 piston is up, may be so small as to lie in the space 

 between the two valves when the piston is down, 

 without being sufficiently condensed for its spring 

 to overcome the adhesion of the bladder forming 

 the upper valve, and the weight of the atmosphere 

 that presses it : in this case, the upper valve will 

 remain shut, and the exhaustion cannot proceed. 



Various modern improvements in the air-pump 

 obviate these inconveniences in a great degree. 

 Mr. Smeaton enlarged the size of the lower valve ; 

 and, to strengthen it, supported it on a brass 

 grating, resembling a honeycomb. This allowed 

 the valve to rise more easily. He also covered the 

 top of the barrel, making the piston-rod work 

 through a collar of leathers ; by which he took off 

 the pressure of the atmosphere from the piston- 

 valve, which acted against the rarefied air in the 

 receiver. Pumps on this construction have been 

 made by Nairne, and other instrument-makers, and 

 have answered extremely well. 



It would not come within the limits of this work 

 to enumerate all the improvements, and different 

 modes of construction, used in this instrument. 

 The latest are the air-pumps, made by Haas and 

 Hurter, Cuthbertson, and Prince ; each of which 

 has particular advantages. 



THE AIR GUN. 



This pneumatical instrument is an ingenious 

 contrivance, which will drive a bullet with great 



