138 THE WORLD-ENERGY 



The relation of attraction, with its manifestation of 

 mutual approach, is not less real, as between fhe merest 

 mote on the one hand, and the earth's mass on the 

 other, than between the earth and the moon, or between 

 the units composing a group known as a double star, 

 where such relation, as exhibited in the revolution of 

 the bodies round each other, is so immeasurably more 

 conspicuous. By a "scientific" fiction, we attribute all 

 the motion to one of the bodies and assume that the 

 other is wholly unaffected by the relation. 



It is, indeed, true that in the case of "falling bodies" 

 this does not affect the accuracy of the results, so far 

 as external measurement is concerned. But it cannot 

 fail to vitiate the results more or less seriously, so far as 

 really scientific thinking is concerned. At the least, the 

 notes of caution in this respect ought to be unfailingly 

 given in text-books of physics, and ought to be far 

 more strongly emphasized than is the case where they 

 are given at all at least, if a text-book is to be an 

 instrumentality in mental discipline, and not merely a 

 means toward percentages in examination. 



Finally, before leaving the consideration of the second 

 law of motion, let us note the ultimate implication of 

 the parallelogram of forces as illustrative of that law. 



The case is sufficiently familiar. Any two forces 

 acting (let us here suppose) from different directions, and 

 not in the same straight line, upon one and the same 

 body or force-center, will each produce the same amount 

 of motion in the body, and in the same direction, as if 

 it alone acted upon the body. Thus, by compounding 

 the two motions, we may find at what point the body 

 will be at the end of any given time. 



