Air-Pump. 403 



This influx of air towards the equator appears to be counter- 

 balanced by a current in the higher regions of the atmosphere 

 towards the poles. The heated air at the equator continues to 

 ascend until it reaches an altitude where its elasticity exceeds 

 that of the air which surrounds it. It then begins to diffuse 

 itself towards the poles, maintaining an equilibrium in the atmos- 

 phere, and moderating the intense cold of the climates towards 

 which it is wafted. Such would be, perhaps, the only fluctua- 

 tions in the atmosphere were the whole globe covered with water, 

 or the variations of heat in the earth's surface regular and con- 

 stant, so as to be the same every where over the same parallel 

 of latitude. As it is, however, we find the irregularities of heat, 

 arising from the interspersion of sea and land, to be such that 

 though all the parts of the atmosphere in some sort conspire to 

 produce regular winds round the torrid zone, yet the effect of 

 land is so great, that striking inequalities are produced ; witness the 

 monsoons, the sea and land breezes, &c., which can be accounted 

 for upon no other principle than that of rarefaction, because the 

 rotatory velocity of different parallels in the torrid zone is nearly 

 alike. 



On the Theory of the Air Pump, and Pumps for raising Water, 



507. THE Air-pump is a machine fitted to exhaust the air 

 from a proper vessel, and thus to produce what is called a va- 

 cuum ; it is one of the most useful of philosophical experiments. 

 By means of it the chief propositions relative to the weight and 

 elasticity of the air are proved experimentally, in a simple and 

 saiisfactory manner. 



EFGH represents a square table of wood, Ji, A two strong bar- Fig. 244. 

 rels or tubes of brass, firn.ly retained in their position by the 

 cross-piece T J 1 , which is pressed on them by screws O, O, fixed on 

 the tops of the brass pillars JV, N. These barrels communicate 

 with a cavity in the lower part D of the table. At the bottom 

 within each barrel is fixed a valve, opening upwards ; and in 

 each barrel a piston works, having a valve likewise opening up- 



