Vital Physics. 51 



But let the subject of attraction being directly as the 

 mass, be first examined. 

 Airy thus speaks of it : 



" There is only one more point regarding the law of 

 gravitation, on which I will here speak ; it is the velocity or 

 the change of motion which an attractive body produces on 

 another body. I have spoken of attraction as if it were 

 directed towards the sun, but we shall find that experiments 

 of various kinds lead us to this conclusion that every 

 particle of matter attracts every other particle of matter, 

 and that every planet attracts every other planet, that 

 every planet attracts the sun, that the sun attracts the 

 planets, and that the sun attracts the moon, and the moon 

 attracts the sun, and that every body attracts every other 

 body. Now the thing I wish you to understand is this : 

 Suppose Venus and the sun are at equal distances from the 

 earth, then the earth pulls the sun out of its way, just as 

 much as it pulls Venus out of the way; the enormous 

 difference of magnitude of the attracted bodies makes no 

 difference in the movement which the action of the 

 attracting body produces on them. If there are two bodies, 

 a great one and a little one, and if something else attracts 

 them, the great body is pulled through as many feet or 

 miles in an hour as the little one."* 



Perhaps there is no subject more difficult to comprehend 

 than attraction being directly as the mass ; the general 

 impression is that attraction is directly in proportion to the 

 mass, but to suppose the earth to pull the sun as much out 

 of his way as the sun does the earth, at once dispels the 

 illusion. 



* Airy's " Lectures on Astronomy," delivered at Ipswich, 1848, 

 fourth edition, page 106. 



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