USE OF GRAVITATION. 117 



gravitation of distance is not affected by the spe- 

 cific gravity ; for if it were not that the air resists 

 the one more than it resists the other, because there 

 is more weight of air opposed to the same weight 

 of it, a cobweb would fall as fast and as straight 

 from the top of St. Paul's, or any other height, as 

 a mill-stone. 



But though the place of no substance can be even 

 in the least changed without a change in the gravi- 

 tation of distance, yet the alterations produced by 

 small changes at long distances are very small. 

 The distance of the mean surface of the earth from 

 its centre is about four thousand miles, and the 

 highest mountain known is less than five miles 

 more, so that the weight of an ordinary man *vould 

 not be half a pound less if he were on the top of the 

 mountain than if he were on the seashore. Sub- 

 stances that have elasticity or spring in them, so 

 that they are affected by pressure, show even a 

 much less elevation than that. The air is altogether 

 a spring, pressed down by its own weight, and it 

 shows the changes of height very nicely. Water 

 boils with less heat too; and the human body is 

 affected, the air which is with the blood or other 

 liquids, in the very small vessels, under delicate skin, 

 swells the vessels, and sometimes bursts them and 

 they bleed. 



Gravitation is the grand principle by which Na- 

 ture, on the great scale, is held together ; and, under 

 the same circumstances, it is uniform in its appa- 

 rent effects. In itself it is always the same, follow- 

 ing matter through all its changes, dead or alive, at 

 rest or in motion. The paper of this book has, in 

 the same identical particles which now form it, beon 

 changed from one visible substance to another, prob- 

 ably millions of times since it was created ; and it 

 has very likely been scattered through millions of 

 substances at the same instant ; but not one atom 

 of it has been lost ; and in all its changes the amount 



