THE FOUR EXPERIMENTAL METHODS. 475 



we do not know the total quantities, we cannot, of 

 course, determine the real numerical relation according 

 to which those quantities vary. It is therefore an 

 error to conclude, as some have concluded, that be- 

 cause increase of heat expands bodies, that is, increases 

 the distance between their particles, therefore that 

 distance is wholly the effect of heat, and that if we 

 could entirely exhaust the body of its heat, the par- 

 ticles would be in complete contact. This can never 

 be 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 diminution of the quantity 

 of heat according to such a numerical relation 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 

 accordingly the ancients, with their inductio per enu- 

 merationem simplicem, 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 



