THE FOUR EXPERIMENTAL METHODS. 445 



body of its heat, the particles would be in complete contact. 

 This is no more than a guess, and of the most hazardous sort, 

 not a legitimate induction : for since we neither know how 

 much heat there is in any body, nor what is the real distance 

 between any two of its particles, we cannot judge whether the 

 contraction of the distance does or does not follow the diminu 

 tion of the quantity of heat according to such a numerical re 

 lation that the two quantities would vanish simultaneously. 



In contrast with this, let us consider a case in which the 

 absolute quantities are known ; the case contemplated in the 

 first law of motion ; viz. that all bodies in motion continue to 

 move in a straight line with uniform velocity until acted upon 

 by some new force. This assertion is in open opposition to 

 first appearances ; all terrestrial objects, when in motion, 

 gradually abate their velocity and at last stop ; which accord 

 ingly the ancients, with their inductio per enumerationem sim- 

 plicem, imagined to be the law. Every moving body, however, 

 encounters various obstacles, as friction, the resistance of the 

 atmosphere, &c., which we know by daily experience to be 

 causes capable of destroying motion. It was suggested that 

 the whole of the retardation might be owing to these causes. 

 How was this inquired into ? If the obstacles could have 

 been entirely removed, the case would have been amenable to 

 the Method of Difference. They could not be removed, they 

 could only be diminished, and the case, therefore, admitted 

 only of the Method of Concomitant Variations. This accord 

 ingly being employed, it was found that every diminution of 

 the obstacles diminished the retardation of the motion : and 

 inasmuch as in this case (unlike the case of heat) the total 

 quantities both of the antecedent and of the consequent were 

 known ; it was practicable to estimate, with an approach to 

 accuracy, both the amount of the retardation and the amount 

 of the retarding causes, or resistances, and to judge how near 

 they both were to being exhausted ; and it appeared that the 

 effect dwindled as rapidly, and at each step was as far on the 

 road towards annihilation, as the cause was. The simple 

 oscillation of a weight suspended from a fixed point, and moved 

 a little out of the perpendicular, which in ordinary circum- 



