360 INDUCTION. 



ticular case. 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 on circumstantial evidence. We can 

 conclude that a man was murdered, though it is not proved by the testimony 

 of eye-witnesses that some person who had the intention of murdering him 

 was present on the spot. It is enough for most purposes, if no other known 

 cause could have generated the efEects shown to have been produced. 



The celebrated speculation of Laplace concerning the origin of the earth 

 and planets, participates essentially in the 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 ; from 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 gravitation, has caused the sun to 

 abandon successive rings of vaporous matter, which are supposed to have 

 condensed by cooling, and to have become the planets. There is in this 

 theory no unknown substance introduced on supposition, nor any unknown 

 property or law ascribed to a known substance. The known laws of mat- 

 ter authorize us to suppose that a body which is constantly giving out so 

 large an amount of heat as the sun is, must be progressively cooling, and 

 that, by the process of cooling it must contract ; if, therefore, we endeavor, 

 from the present state of that luminary, to infer its state in a time long 

 past, we must necessarily suppose that its atmosphere extended much far- 

 ther than at present, and we are entitled to suppose that it extended as far 

 as we can trace effects such as it might naturally leave behind it on retir- 

 ing ; and such the planets are. These suppositions being made, it follows 

 from known laws that successive zones of the solar atmosphere might be 

 abandoned; that these would continue to revolve round the sun with the 

 same velocity as when they formed part of its substance ; and that they 

 would cool down, long before the sun itself, to any given temperature, and 

 consequently to that at which the greater part of the vaporous matter of 

 which they consisted would become liquid or solid. The known law of 

 gravitation would then cause them to agglomerate in masses, which would 

 assume the shape our planets actually exhibit ; would acquire, each about 

 its own axis, a rotatory movement; and would in that state revolve, as the 

 planets actually do, about the sun, in the same direction with the sun's ro- 

 tation, but with less velocity, because in the same periodic time which the 

 sun's rotation occupied when his atmosphere extended to that point. There 

 is thus, in Laplace's theory, nothing, strictly speaking, hypothetical ; it is 

 an example of legitimate reasoning from a present effect to a possible past 

 cause, according to the known laws of that cause. The theory, therefore, 

 is, as I have said, of a similar character to the theories of geologists ; but 

 considerably inferior to them in point of evidence. Even if it were proved 

 (which it is not) that the conditions necessary for determining the break- 

 ing off of successive rings would certainly occur, there would still be a 

 much greater chance of error in assuming that the existing laws of nature 

 are the same which existed at the origin of the solar system, than in mere- 

 ly presuming (with geologists) that those laws have lasted through a few 

 revolutions and transformations of a single one among the bodies of which 

 that system is composed. 



