262 



INDUCTION. 



to heat as a cause. But we have 

 still a resource. Though we cannot 

 exclude an antecedent altogether, we 

 may be able to produce, or nature 

 may produce for us, some modification 

 in it. By a modification is here 

 meant a change in it, not amounting 

 to its total removal. If some modi- 

 fication in the antecedent A is always 

 followed by a change in the conse- 

 quent a, the other consequents 6 and 

 c remaining the same ; or vice versd, 

 if every change in a is found to have 

 been preceded by some modification 

 in A, none being observable 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 can- 

 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 obser- 

 vation already treated of, that such 

 increase or diminution of heat is fol- 

 lowed by expansion or contraction of 

 the body. In this manner we arrive 

 at the conclusion, otherwise unattain- 

 able 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 sur- 

 face 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 j but when we find that 



all the variations in the position of 

 the moon are followed by correspond- 

 ing 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 re- 

 mote 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 efiEect are correspon- 

 dent, 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 in- 

 dispensable 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 pendu- 

 lum 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 perpendicular 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 characterised that all 

 terrestrial bodies tend to the earth, 

 and not to some unknown fixed point 

 lying in the same direction. In every 

 twenty-four hours, by the earth's 

 rotation, the line drawn from 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 two hundred millions 

 of miles ; yet in all these changes of 

 the earth's position, the line in which 

 bodies tend to fall continues to be 

 directed towards it : which proves 

 that terrestrial gravity is directed 

 to the earth, and not, as was once 



