i 9 4 PHYSICAL SCIENCE BK. v 



be : a wave is an agitation of the sea in one 

 direction. So in the subject which at the moment 

 forms our special topic, the definition will not be 

 unduly restricted if one is careful to say : wind 

 is the atmosphere flowing in one direction ; or, wind 

 is atmosphere flowing through some impulse, or, is 

 the force of the atmosphere going in one direction, 

 or, is a rush of the atmosphere more forcible than 

 usual in some one direction. I am aware of a 

 criticism that may be made in regard to the first 

 4 definition. What need is there to add that it is in 

 one direction that the atmosphere flows ? For 

 surely whatever flows, flows in one direction. No 

 one says that water flows if there is simply an 

 internal movement of it, but only if it is borne in 

 a particular direction. So a substance may be in 

 motion and yet not flow ; but, on the other hand, it 

 cannot flow except in one direction. Well, if, on 

 the one hand, the shorter definition is free from cavil, 

 let us employ it ; but if, on the other, any one is a 

 stickler, let him not omit the phrase whose addition 

 will serve to preclude all ambiguity. Now that we 

 have sufficiently discussed our terms, let us come to 

 grapple with our problem at closer quarters. 



II 



DEMOCRITUS avers that when there are many 

 particles, which he calls atoms, in a small empty 

 space (i.e. vacuum), wind is the outcome. But, on the 

 contrary, when the space is large and the particles 

 few, there is a still peaceful condition of the atmo- 

 sphere. To illustrate : in the market square or in 

 a side street as long as there is a sprinkling of 



