THE ATMOSPHERE. 



below it, being pressed upon by the weight of only one-i 

 would have a thickness of 40 feet, and so on. 



The air, therefore, in ascending, becomes continually and 

 indefinitely thinner and rarer. Persons who have ascended to 

 great heights in balloons or on mountains, have accordingly found 

 themselves in an atmosphere so rarefied as to derange seriously 

 the vital functions. 



16. The various phenomena vulgarly called suction are nothing 

 more than so many various effects of the atmospheric pressure. 



17. If a piece of moist leather be placed in close contact with 

 any heavy body having a smooth surface, such as a stone or a 

 piece of metal, it will adhere to it ; and if a cord be attached to 

 the leather, the stone or metal may be raised by it. 



This effect arises from the exclusion of the air from between 

 the leather and the stone. The weight of the atmosphere presses 

 their surfaces together with a force amounting to 15 Ib. on a 

 square inch of the surface of contact. 



18. The power of flies, and other insects, to walk on ceilings, 

 smooth pieces of wood, and other similar surfaces, in doing which 

 the gravity of their bodies appears to have no effect, is explained 

 upon the same principle. Their feet are provided with an appa- 

 ratus similar exactly to the leather applied to the stone. 



19. The pressure and elasticity of the air are both called into 

 effect in the act of respiration. When we inspire the atmosphere, 

 we make by a muscular exertion an enlarged space within the 

 chest. The atmospheric pressure forces the external air into this 

 space. By another and contrary muscular exertion, the chest is 

 then contracted, so as to squeeze out the air which has been 

 inhaled, and which, by compression, acquires an elasticity greater 

 than the atmospheric pressure, in virtue of which it is forced out 

 at the mouth and nostrils. 



20. The action of a common bellows is precisely similar, except 

 that the aperture through which the air enters is different from 

 that by which it is expelled. The analogy, however, would be 

 complete if we inspired by the mouth and expelled by the nose. 

 When the boards of the bellows are separated, the inner chamber 

 is enlarged, and the air is forced in by the external pressure 

 through the aperture governed by the leather valve or clack. 

 The boards being then pressed together, and the escape of the 

 air being stopped by the closed valve, it is compressed until it 

 acquires an elasticity greater than the atmospheric pressure, and 

 is forced out. 



Bellows on a large scale are constructed with an intermediate 

 board, so as to consist of two chambers, and to produce a con- 

 tinued instead of an intermittent blast. This is nothing more 

 106 



