318 THE MECHANICAL EQUIVALENT OF HEAT. 



The rule which must be followed, in order to lay the foun- 

 dations of a knowledge of nature in the shortest conceivable 

 time, may be comprised in a few words. The natural phe- 

 nomena with which we come into most immediate contact, 

 and which are of most frequent occurrence, must be subjected 

 to a careful examination by means of the organs of sense, 

 and this examination must be continued until it results in 

 quantitative determinations which admit of being expressed 

 by numbers. 



These numbers are the required foundations of an exact 

 investigation of nature. 



Among all natural operations, the free fall of a weight is 

 the most frequent, the simplest, and witness Newton's apple 

 at the same time the most important. When this process 

 is analysed in the way that has been mentioned, we imme- 

 diately see that the weight strikes against the ground the 

 harder the greater the height from which it has fallen ; and 

 the problem now consists in the determination of the quanti- 

 tative relations subsisting between the height from which the 

 weight falls, the time occupied by it in its descent, and its 

 final velocity, and in expressing these relations by definite 

 numbers. 



In carrying out this experimental investigation, various 

 difficulties have to be contended with ; but these must and 

 can be overcome ; and then the truth is arrived at, that for 

 every body a fall of sixteen feet, or a time of descent of one 

 second, corresponds to a final velocity of thirty-two feet per 

 second. 



A second phenomenon of daily occurrence, which is in 

 apparent contradiction to the laws of falling bodies, is the 

 ascent of liquids in tubes by suction. Here, again, the rule 

 applies, not to allow the maxim, velle rerum cognoscere causas, 

 to lead us into error through useless and therefore harmful 

 speculations concerning the qualities of the vacuum, and the 

 like ; on the contrary, we must again examine the phenome- 



