HYPOTHESES. 201 



known class of plicnomena. An instance of the first kind is afforded 

 by the dift'ercnt suppositions respecting the law of the planetary cen- 

 tral force, anterior to the discovery of the true law, that the force vaines 

 as the inverse square of the distance ; which was itself suggested by 

 Newton, nn the first instance, as an hypothesis, and was verified by 

 proving that it led deductively to Kepler's laws. Hypotheses of the 

 second kind are such as the vortices of Descartes, which were fictitious, 

 but were supposed to obey the known laws of rotatory motion ; or the 

 two rival hypotheses respecting the nature of light, the one ascribing 

 the phenomena to a fluid emitted fiom all luminous bodies, the other 

 (now more generally received) attributing them to vibratory motions 

 among the particles of an ether peiTading all space. Of the existence 

 of either fluid there is no evidence, save the explanation they are cal- 

 culated to afford of some of the phenomena; but they are supposed to 

 produce their effects according to known laws ; the ordinary laws of 

 continued locomotion in the one case, and in the other, those of the 

 propagation of undulatory movements among the particles of an elastic 

 fluid. 



According to the foregoing remarks, hypotheses are invented to en- 

 able the Deductive Method to be earlier applied to phenomena. But* 

 in order to discover the cause of any phenomena by the Deductive 

 Method, the process mTist consist of three parts ; induction, ratiocin- 

 ation, and verification. Induction, (the place of which, however, may 

 be supplied by a prior deduction,) to ascertain the laws of the causes ; 

 ratiocination, to compute from those laws, how the causes will oper- 

 ate in the particular combination known to exist in the case in hand ;• 

 verification, by comparing this calculated effect with the actual phe- 

 nomenon. No one of these three parts of the process can be dis- 

 pensed with. In the great deduction which proves the identity of 

 gravity and the central force of the solar system, all the. three are 

 found. First, it is proved from the moon's motions, that the earth 

 a;ttracts her with a force varying as the inverse square of the distance. 

 This (though partly dependent on prior deductions) con-esponds to 

 ' the first, or purely inductive step, the ascertainment of the law of the 

 cause. Secondly, fi-om this law, and from the knowledge previously 

 obtained of the moon's mean distance from the earth, and of the actual 

 amount of her deflexion fi'om the tangent, it is ascertained with what 

 rapidity the earth's attraction would cause her to fall, if she were no 

 further off*, and no more acted upon by extraneous forces, than teiTes- 

 ti'ial bodies are: this is the second step,- the ratiocination. Fing,lly, 

 this calculated velocity being compared with the observed velocity 

 with whicli all heavy bodies fall, by mere gi-aVity, towards the surface 

 of the earth, (namely sixteen feet in the first second, forty-eight in the 

 second, and so forth, in thq^ratio of the odd numbers, 1, 3, 5, &c.,) the 

 two quantities were found to agree. The order in which I liaye here 

 presented the three steps was not the exact -order of their discovery ; 

 but it is their correct logical . order, as portions of the proof that the 

 same attraction of the earth wliich causes the nioon's motion, causes 

 also the fall of heavy bodies to the earth : a, proof which is thus com- 

 plete In all its parts. ' ' 



Now, the Hypothetical Method suppresses the first of the three steps, 



» Vide supra, p. 264. 



