12 INDUCTION. 



known laws ; the ordinary laws of continued locomo- 

 tion 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 enable the Deductive Method to be 

 earlier applied to phenomena. But* in order to 

 discover the cause of any phenomena by the Deduc- 

 tive Method, the process must consist of three parts ; 

 induction, ratiocination, 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 operate 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 dispensed 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 

 attracts her with a force varying as the inverse square 

 of the distance. This (though partly dependent on 

 prior deductions) corresponds to the first, or purely 

 inductive, step, the ascertainment of the law of the 

 cause. Secondly, from this law, and from the know- 

 ledge previously obtained of the moon's mean distance 

 from the earth, and of the actual amount of her 

 deflexion from 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 terrestrial bodies are : this 

 is the second step, the ratiocination. Finally, this 



* Vide supra, vol. i. p. 534. 



