INDUCTION. 



of the term, be called laws of nature. But of the seven, three 

 alone are properly distinct and independent: these being pre 

 supposed, the others follow of course. The three first, there 

 fore, according to the stricter acceptation, are called laws of 

 nature ; the remainder not ; because they are in truth mere 

 cases of the three first ; virtually included in them ; said, there 

 fore, to result from them : whoever affirms those three has 

 already affirmed all the rest. 



To substitute real examples for symbolical ones, the follow 

 ing are three uniformities, or call them laws of nature : the 

 law that air has weight, the law that pressure on a fluid is 

 propagated equally in all directions, and the law that pressure 

 in one direction, not opposed by equal pressure in the contrary 

 direction, produces motion, which does not cease until equili 

 brium is restored. From these three uniformities we should 

 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 sus 

 tained 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 pressure on the mercury not being pro 

 pagated equally in an upward 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 experi 

 ment, we might deduce its result from those laws. The known 

 weight of the air, combined with the position of the appa 

 ratus, 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 characterized in treating of Ratiocination. We should 

 thus come to know the more complex uniformity, indepen- 



