440 INDUCTION. 



not expel it altogether from any body, we can modify it in 

 quantity, we can increase or diminish it ; and doing so, we 

 find by the various methods of experimentation or observation 

 already 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 

 variable relations to other things, of which variable 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 the part of the earth which is nearest to, or that 

 which is most remote from, the moon, we have ample evidence 

 that the moon is, wholly or partially, the cause which deter 

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

 instance, that the variations of an effect are correspondent, or 

 analogous, to those of its cause ; as the moon moves farther 

 towards the east, the high water point does the same : but this 

 is not an indispensable 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, of necessity, moves towards the 

 west, as the moon, followed by the nearer of the tide waves, 

 advances towards the east : and yet both these motions are 

 equally effects of the moon s motion. 



That the oscillations of the pendulum are caused by the 

 earth, is proved by similar evidence. Those oscillations take 



