274 PHYSIOLOGICAL PHYSICS. [Chap. xxm. 



of gas as made up of layers, then the topmost layer of 

 gas will exert a pressure equal to its own weight ; the 

 second layer will exert a pressure equal to its own 

 weight plus the pressure of the layer above it. Thus 

 the body will come to support a pressure equal to that 

 exerted by the layers above it over an extent of 

 surface equal to its area. In other words, the weight 

 on the body will be equal to a column of gas whose 

 base is the surface of the body and whose height is the 

 distance between the body and the surface of the 

 gaseous mass. Just as in liquids, also, the body will 

 be pressed upwards by a force equal to the weight of 

 gas which it displaces. The weight of a body in air, 

 therefore, is not its true weight, but only the differ- 

 ence between the true weight and the weight of the 

 displaced volume of air. 



Atmospheric pressure. A mass of air (the 

 atmosphere) completely surrounds the earth. In 

 accordance with what has been already stated, it will 

 exert" pressure in all directions, and the pressure will 

 vary according to the thickness of the mass. The 

 pressure exerted by the atmosphere on any body will, 

 therefore, dimmish as the body rises in the air. The 

 pressure on the top of a hill is less than in the valley. 

 Gas is compressible, and since the lowest layers of the 

 atmospheres will sustain the pressure of all the layers 

 above them, they will be very much compressed arid 

 consequently more dense. The density will thus be 

 greatest on the surface of the earth, and will diminish 

 with the distance from the surface. Suppose the 

 density had been uniform, then a layer of air about 

 five miles in thickness encircling the earth would give 

 a pressure equal to the ordinary atmospheric pressure. 

 This height (five miles) is called the height of the 

 homogeneous atmosphere. The constantly diminishing 

 density as one ascends necessitates a much greater 

 thickness of layer to give the pressure. The real 



