VARIATIONS OF GRAVITY. 105 



or one-half time thirty-two, or one-quarter time 

 sixty-four or, in short, some product of any num- 

 ber that can possibly be named. There are vari- 

 ous natural causes that alter the specific gravity 

 of substances, though more extensively in some 

 than in others ; and some of the most important, 

 as well as the most curious results and appear- 

 ances in nature are owing to those changes. In 

 dry wood, the changes of specific gravity, in the 

 same piece, are very small ; while, in the common 

 air that we breathe, they are great ; and in coun- 

 tries and at seasons that have the weather vari- 

 able, they are constantly taking place. They are, 

 indeed, among the immediate causes of some of 

 the changes in the weather, and they are in other 

 cases the effects on that subject it is not very 

 easy to distinguish between causes and effects. 

 In the events themselves, there is no difference 

 between what we call causes, and what we call 

 effects ; for every cause is the effect of a former 

 cause, whether that cause be known to us or not ; 

 and every effect is the cause of a future effect, 

 whether or not we shall discover or otherwise know 

 that effect. 



The only other variation of gravity which it is 

 necessary to mention, for purposes so very general 

 as ours, is the variation of its intensity with change 

 of distance. The farther any one piece of matter 

 is removed from any other piece, the less does the 

 one gravitate towards the other. If, for instance, 

 one body be removed to twice the distance from 

 another, its gravitating tendency will be only one- 

 fourth of what it was before ; and if it be brought 

 to one-third of its former distance, it will gravi- 

 tate nine times as much. It is usual to say that 



