HYPOTHESES. 359 



of causation has molecular motion both at the beginning and end. The 

 hypothesis only makes the motion continuous by extending it to the mid- 

 dle. Now, motion in a body is known to be capable of being imparted to 

 another body contiguous to it ; and the intervention of a hypothetical elas- 

 tic fluid occupying the space between the sun and the earth, supplies the 

 contiguity Avhich is the only condition wanting, and which can be supplied 

 by no supposition but that of an intervening medium. The supposition, 

 notwithstanding, is at best a probable conjecture, not a proved truth. For 

 there is no proof that contiguity is absolutely required for the communica- 

 tion of motion from one body to another. Contiguity does not always ex- 

 ist, to our senses at least, in the cases in which motion produces motion. 

 The forces which go under the name of attraction, especially the greatest 

 of all, gravitation, are examples of motion producing motion without ap- 

 parent contiguity. When a planet moves, its distant satellites accompany 

 its motion. The sun carries the whole solar system along with it in the 

 progress which it is ascertained to be executing through space. And even 

 if we were to accept as conclusive the geometrical reasonings (strikingly 

 similar to those by which the Cartesians defended their vortices) by which 

 it has been attempted to show that the motions of the ether may accoimt 

 for gravitation itself, even then it would only have been proved that the 

 supposed mode of production may be, but not that no other mode can be, 

 the true one. 



§ 1. It is necessary, before quitting the subject of hypotheses, to guard 

 against the appearance of reflecting upon the scientific value of several 

 branches of physical inquiry, which, though only in their infancy, I hold to 

 be strictly inductive. There is a great difference between inventing agen- 

 cies to account for classes of phenomena, and endeavoring, in conformity 

 with known laws, to conjecture what former collocations of known agents 

 may have given birth to individual facts still in existence. The latter is 

 the 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 which we have actual experience of its origin. This, for ex- 

 ample, is the scope of the inquiries of geology ; and they are no more illogic- 

 al 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 

 on uniformities ascertained 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 mat- 

 ter 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 now produced, we may rationally, and without hypothesis, con- 

 clude either that the causes existed formerly with greater intensity, or that 

 they have operated during an enormous length of time. 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 though the laws 

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

 known agents, those agents are not known to have been present in the par- 



