VARIATIONS OF GRAVITY. 115 



two, or one-quarter time sixty-four or, in short, 

 some product of any number that can possibly be 

 named. There are various natural causes that alter 

 the specific gravity of substances, though more ex- 

 tensively in some than in others; and some of the 

 most important, as well as the most curious results 

 and appearances 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 countries and at seasons that have the wea- 

 ther variable, 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 gravitate 

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

 gravitate towards each other inversely as the squares 

 of their distances, that is, if one body is at the 

 distance two, and another at the distance three, the 

 body at two will, on account of distance, gravitate 

 as nine, while the body at three, gravitates as four. 



