LAWS OF NATURE. 



207 



be able to predict another uniformity, 

 namely, the rise of the mercury in the 

 Torricellian tube. This, in the stricter 

 use of the phrase, is not a law of nature. 

 It is the result of laws of nature. It 

 is a case of each and every one of the 

 three laws ; and is the only occurrence 

 by which they could all be fulfilled. 

 If the mercury were not sustained in 

 the barometer, and sustained at such 

 a height that the column of mercury 

 were equal in weight to a column of 

 the atmosphere of the same diameter ; 

 here would be a case, either of the air 

 not pressing upon the surface of the 

 mercury with the force which is called 

 its weight, or of the downward pres- 

 sure on the mercury not being propa- 

 gated equally in an upper direction, 

 or of a body pressed in one direction 

 and not in the direction opposite, 

 either not moving in the direction in 

 which it is pressed, or stopping before 

 it had attained equilibrium. If we 

 knew, therefore, the three simple laws, 

 but had never tried the Torricellian 

 experiment, we might deduce its re- 

 sult from those laws. The known 

 weight of the air, combined with the 

 position of the apparatus, would bring 

 the mercury within the first of the 

 three inductions ; the first induction 

 would bring it within the second, and 

 the second within the third, in the 

 manner which we characterised in 

 treating of Ratiocination, We should 

 thus come to know the more complex 

 uniformity, independently of specific 

 experience, through our knowledge of 

 the simpler ones from which it results ; 

 though, for reasons which will appear 

 hereafter, verification by specific expe- 

 rience would still be desirable, and 

 might possibly be indispensable. 



Complex uniformities which, like 

 this, are mere cases of simpler ones, 

 and have, therefore, been virtually 

 affirmed in aflarming those, may with 

 propriety be called laws, but can 

 scarcely, in the strictness of scientific 

 speech, be termed Laws of Nature. 

 It is the custom in science, wherever 

 regularity of any kind can be traced, 

 to call the general proposition which 



expresses the nature of that regularity 

 a law ; as when, in mathematics, we 

 speak of the law of decrease of the 

 successive terms of a converging 

 series. But the expression law of 

 nature has generally been employed 

 with a sort of tacit reference to the 

 original sense of the word law, namely, 

 the expression of the will of a superior. 

 When, therefore, it appeared that any 

 of the uniformities which were ob- 

 served in nature would result spon- 

 taneously from certain other unifor- 

 mities, no separate act of creative 

 will being supposed necessary for the 

 production of the derivative unifor- 

 mities, these have not usually been 

 spoken of as laws of nature. Accord- 

 ing to one mode of expression, the 

 question. What are the laws of nature ? 

 may be stated thus : What are the 

 fewest and simplest assumptions, 

 which being granted, the whole exist- 

 ing order of nature would result? 

 Another mode of stating it would be 

 thus : What are the fewest general 

 propositions from which all the uni- 

 formities which exist in the universe 

 might be deductively inferred ? 



Every great advance which marks 

 an epoch in the progress of science 

 has consisted in a step made towards 

 the solution of this problem. Even a 

 simple colligation of inductions already 

 made, without any fresh extension 

 of the inductive inference, is already 

 an advance in that direction. When 

 Kepler expressed the regularity which 

 exists in the observed motions of the 

 heavenly bodies by the three general 

 propositions called his laws, he, in so 

 doing, pointed out three simple sup- 

 positions, which, instead of a much 

 greater number, would suffice to con- 

 struct the whole scheme of the hea- 

 venly motions so far as it was known 

 up to that time. A similar and still 

 greater step was made when these 

 laws, which at first did not seem to 

 be included in any more general 

 ti-uths, were discovered to be cases of 

 the three laws of motion, as obtain- 

 ing among bodies which mutually 

 tend towards oae another with » cer- 



