THE CONQUEST OF NATURE 



and farther away from its present line of elliptical flight. 

 Or if, on the other hand, the persistent force were ap- 

 plied from the side opposite the sun, it would suffice 

 ultimately to carry the earth in a spiral course until it 

 plunged into the sun itself. Indeed it has been ques- 

 tioned in modern times whether it may not be possible 

 that precisely this latter effect is gradually being 

 accomplished, through the action of meteorites, some 

 millions of which fall out of space into the earth's 

 atmosphere every day. If these meteorites were 

 uniformly distributed through space and flying in 

 every direction, the fact that the sun screens the earth 

 from a certain number of them, would make the aver- 

 age number falling on the side away from the sun 

 greater, and thus would in the course of ages produce 

 the result just suggested. All that could save our earth 

 from such a fate would be the operation of some coun- 

 teracting force. Such a counteracting force is perhaps 

 found in solar radiation. It may be added that the 

 distribution of meteorites in space is probably too 

 irregular to make their influence on the earth predicable 

 in the present state of science ; but the principle involved 

 is no less sure. 



WHEELS AND PULLEYS 



Returning from such theoretical applications of the 

 principle of motion, to the practicalities of e very-day 

 mechanisms, we must note some of the applications 

 through which the principle of the lever is made avail- 

 able. Of these some of the most familiar are wheels, and 

 the various modifications of wheels utilized in pulleys 



[32] 



