THE FOUR EXPERIMENTAL METHODS. 287 



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 meth- 

 ods of experimentation or observation ah-eady 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 va- 

 riable relations the principal is its position in space. In the previous ex- 

 ample, the modification which was produced in the antecedent was an al- 

 teration in its quantity. Let us now suppose the question to be, what in- 

 fluence the moon exerts on the surface of the earth. We can not 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 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 hap- 

 pens, 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 farther 

 toward 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 toward 

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

 toward 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 perpendicular to the earth, 

 varies with every variation in the eai'th's position, either in space or rela- 

 tively to the object. Speaking accurately, we only know by the method 

 now characterized, 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 toward it: 

 which proves that terrestrial gravity is directed to the earth, and not, as 

 was once fancied by some, to a fixed point of space. 



The method by which these results were obtained may be termed the 

 Method of Concomitant Variations ; it is regulated by the following canon : 



Fifth Canon. 



Whatever phenomenon varies in any manner whenever another phe- 

 nomenon varies in some particular m,anner^ is either a cause or an effect 

 of that phenomenon, or is connected with it through some fact of cav.sa- 

 tion. 



