THE FOUR EXPERIMENTAL METHODS. 441 



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 characterized, that all terrestrial bodies tend to the earth, 

 and not to some unknown fixed point lying in the same direc 

 tion. 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 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 phenomenon varies in some particular manner, is 

 either a cause or an effect of that phenomenon, or is connected 

 with it through some fact of causation. 



The last clause is subjoined, because it by no means follows 

 when two phenomena accompany each other in their variations, 

 that the one is cause and the other effect. The same thing 

 may, and indeed must happen, supposing them to be two dif 

 ferent effects of a common cause : and by this method alone it 

 would never be possible to ascertain which of the suppositions 

 is the true one. The only way to solve the doubt would be 

 that which we have so often adverted to, viz. by endeavouring 

 to ascertain whether we can produce the one set of variations 

 by means of the other. In the case of heat, for example, by 

 increasing the temperature of a body we increase its bulk, but 

 by increasing its bulk we do not increase its temperature ; on 

 the contrary, (as in the rarefaction of air under the receiver of 

 an air-pump,) we generally diminish it : therefore heat is not 



