THE FOUR EXPERIMENTAL METHODS. 469 



treated of, that such increase or diminution of heat is 

 followed by expansion or contraction of the body. 

 In this manner we arrive at the conclusion, otherwise 

 unattainable by us, that one of the effects of heat is 

 to enlarge the dimensions of bodies ; or what is the 

 same thing in other words, to widen the distances 

 between their particles. 



A change in a thing, not amounting to its total 

 removal, that is, a change which leaves it still the 

 same thing it was, must be a change either in its 

 quantity, or in some of its relations to other things, of 

 which relations the principal is its position in space. 

 In the previous example, the modification which was 

 produced in the antecedent was an alteration in its 

 quantity. Let us now suppose the question to be, 

 what influence the moon exerts on the surface of 

 the earth. We cannot try an experiment in the 

 absence of the moon, so as to observe what terrestrial 

 phenomena her annihilation would put an end to ; 

 but when we find that all the variations in the position 

 of the moon are followed by corresponding variations 

 in the time and place of high water, the place being 

 always either on the side of the earth which is 

 nearest to, or on that which is most remote from, the 

 moon, we have ample evidence that the moon is, 

 wholly or partially, the cause which determines the 

 tides. It very commonly happens, as it does in this 

 instance, that the variations of an effect are corre- 

 spondent, or analogous, to those of its cause ; as the 

 moon moves further towards the east, the high 

 water point does the same : but this is not an indis- 

 pensable condition; as may be seen in the same 

 example, for along with that high water point, there 

 is at the same instant another high water point 

 diametrically opposite to it, and which, therefore., 



