i 4 CORRELATION OF PHYSICAL FORCES. 



In books which treat of statics and dynamics, it is common 

 and perhaps necessary to isolate the subjects of consideration ; 

 to suppose, for instance, two bodies gravitating, and to ignore 

 the rest of the universe. But no such isolation exists in 

 reality, nor could we predict the result if it did exist. Would 

 two bodies gravitate towards each other in empty space, if 

 space can be empty ? The notion that they would is founded 

 on the theory of attraction, which Newton himself repudiated, 

 further than as a convenient means of regarding the subject. 

 For purposes of instruction or argument it may be convenient 

 to assume isolated matter: many conclusions so arrived at 

 may be true, but many will be erroneous. 



If, in producing effects of tension or of static force, the 

 effort made pervades the universe, it may be said, when the 

 bent spring is freed, when the raised weight falls, a converse 

 series of motions must be effected, and this theory would 

 lead to a mere reciprocation, which would be equally unpro- 

 ductive of permanent change with the annihilation of force. 

 If raising the weight has changed the centre of gravity of 

 the earth, and thence of the universe, the fall of the weight, 

 it will be said, restores the original centre of gravity, and 

 everything comes back to its original status. In this argument 

 we again, in thought, isolate our experiment ; we neglect 

 surrounding circumstances. Between the time of the raising 

 and falling of the weight, be the interval never so small, nay 

 more, during the rising and during the fall, the earth has been 

 going on revolving round its axis and round the sun, to say 

 nothing of other changes, such as temperature, cosmical mag- 

 netism, &c., which we may call accidental, but which, if we 

 knew all, would probably be found to be as necessary and as 

 reducible to law as the motion of the earth. A change having 

 taken place, the fall of the weight does not bring back the 

 status quo, but other changes supervene, and so on. Nothing 

 repeats itself, because nothing can be placed again in the 

 same condition : the past is irrevocable. 



