156 THE REALITIES OF MODERN SCIENCE 



irregular natural motions. In the case of wind the 

 motions of the molecules of the air are similar in that 

 they consist of these natural haphazard motions and 

 of a forced motion, essentially the same for all the 

 molecules, which is superimposed upon the natural 

 motion. 



If one thrusts a paper into a swarm of gnats he will 

 recognize that, at whatever angle he may hold it, there 

 will be about as many gnats striking it each moment, 

 for at any instant in their haphazard motions there 

 are about as many flying in any one direction as in any 

 other. Imagine the gnats to be inclosed in a box. 

 There should be about as many striking any square 

 centimeter of its inner wall in a second as there are 

 striking any other equal area. Their impacts, there- 

 fore, should exert the same pressure on all the contain- 

 ing walls. That a large number of such small blows 

 produces the effect of a steady force is well illustrated 

 by the force exerted on an umbrella by the raindrops 

 of a heavy summer shower. About the same number 

 strike each second, and the umbrella must be supported 

 against a constant force. 



Suppose that the volume of the box is halved by 

 pushing the cover down and thus crowding the gnats 

 closer together. They continue to fly as before, but 

 the distance between top and bottom is only half as 

 great, and therefore they require but half the time to 

 make the trip. There should then be twice as many 

 impacts per second as before, and the pressure on the 

 top and bottom should be doubled. 



The area of the sides of the box has been reduced 

 to one half by decreasing the height. Since for each 



