232 INDUCTION. 



by a change in the consequent a, the other consequents h and c re- 

 maining the same ; or, vice versCl, if every change in a is found to have 

 been preceded by some modification in A, none being obsersable in 

 any of the other antecedents ; we may safely conclude that a is, -wholly 

 or in part, an effect traceable to A, or at least in some way connected 

 with it through causation. For example, in the case of heat, though 

 we cannot expel it altogether from any body, we can modify it in quan- 

 tity, we can increase or diminish it ; and doing so, we find by the va- 

 rious methods of experimentation or observation already treated of, 

 that such increase or diminution of heat is followed by expansion or 

 conti-action of the body. In this manner we aiTive at the conclusion, 

 otherwise unattainable by us, that one of the effectsof heat is to enlarge 

 the diiTiensions 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 w'as, 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 teiTestrial phenomena her annihilation would put an end to ; 

 but when we find that all the variations in the j)Ositio7i of the moon are 

 followed by coiTesponding variations in the time and place of high 

 water, the jilace being always either on the side of the earth which is 

 nearest to, or on that which is most remote fi'om, 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 ah effect are coiTespondent, or anal- 

 ogous, 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, 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 place between 

 equidistant points on the two sides of a line, which, being pei-pendic- 

 ular to the earth, varies with every variation in the earth's position, 

 either in space or relatively to the object. Speaking accurately, we 

 only know by the method now charactei-ized, that all terrestrial 

 bodies tend to the earth, and not to some unkno^vn fixed point 

 lying in the same direction. In every twenty-four hours, by the 

 earth's rotation, the line dra\vn fi'om the body at right angles to the 

 earth coincides successively with all the radii of a circle, and in the 

 course of six months the place of that circle varies by nearly tAvo 

 hundred millions ofm.iles; yet in all these changes of the earth's posi- 

 tion, the line in which bodies tend to fall continues to be directed to- 

 wards it : which proves that teiTestrial gravity is directed to the earth, 

 and not, as was once fancied by some, to a fixed point of space. 



