298 INDUCTION. 



tainty of several branches of physical inquiry, which, although only in 

 their infancy, I hold to be strictly inductive. There is a gi'eat differ- 

 ence between inventing laws of nature to account for classes of phe- 

 njomena, and merely endeavoring, in conformity with known laws, to 

 conjecture what collocations, now gone by, may have given birth to 

 individual .facts still in existence. The latter is the strictly legitimate 

 operation of inferring from an observed effect, the existence, in time 

 past, of a cause similar to that by which we know it to be produced in 

 all cases in whiqh we have actual experience of its origin. This, for 

 example, is the scope of the inquiries of geology ; and they ai'e no 

 more, illogical or visionary than judicial inquiries, which also aim at 

 discovering a past event by inference from those of its effects which 

 still subsist. As we can ascertain whether a man was murdered or 

 died a natural death, from the indications exhibited by the corpse, 

 the presence or absence of signs of struggling on the ground or on the 

 adjacent objects, the marks of blood, the footsteps of the supposed 

 murderers, and so on, proceeding throughout upon uniformities ascer- 

 tained- by a perfect induction without any mixture of hypothesis ; so if 

 we find, on and beneath the surface of our planet, masses exactly 

 similar to deposits from water, or to results of the cooling of matter 

 melted by fire^ we may justly conclude that such has been their origin ; 

 and if the effects, though similar in kind, are on a far larger scale than 

 any which are produced now, we may rationally, and without hypoth- 

 esis, conclude that; the causes existed formerly with greater intensity. 

 Further than this no geologist of authority has, since the rise of the 

 present enlightened school of geological speculation, attempted to go. 

 In many geological inquiries it doubtless happens, that although the 

 laws to which the phenomena are ascribed are known laws, and the 

 agents known agents, those agents- are not known to have been pres- 

 ient in the particular case,. Thus in the speculation respecting the 

 igneous origin, of trap or granite, the fact does not admit of direct 

 proof, that those substances have been actually subjected to intense 

 heat. But the same thing might be said of all judicial inquiries -which 

 proceed upon circumstantial evidence. We can conclude that a man 

 was murdered, although it is not proved by the testimony of eye-wit- 

 nesses that a man who had the intention of murdering him was present 

 on the spot. It is enough if no other known cause could have gener- 

 ated the effects shown to have been produced. And so, in geology, 

 it is enough that no other known agent than heat could, according to 

 any known law, have produced the unstratified rocks, while there is 

 the strongest reason to believe that any terrestrial agent capable of 

 operating on so large a scale would not have remained unknown. 



The celebrated speculation of La.place, now very generally received 

 as probable by astronomers, concerning the origin of the earth and 

 planets, participates essentially in the strictly inductive character of 

 modern geological theory. The speculation is, that the atmosphere of 

 the sun originally' extended to the present limits of the solar system; 

 irom which, by the process of cooling, it has contracted to its present 

 dimensions ; and since, by the general principles of mechanics, the 

 rotation of the sun and of its accompanying atmosphere must increase 

 in rapidity as its volume diminishes, the increased centrifugal force 

 generated by the more rapid rotation, overbalancing the action of grav- 

 itation^ would cause the eun to abandon successive rings of vaporous 



