HYPOTHESES. 299 



matter, which are supposed to have condensed by cooling, and to have 

 hecome our planets. There is in this theory no unknown substance 

 introduced ujuin supposition, nor any unknown property or law ascribed 

 to a known substance. Tlic known laws of matter authorize us to 

 suppose that a body which is constantly giving out so large, an amount 

 of heat as the sun is, must be jjrogressively cooling, and that by the 

 process of Qooling it must contract ; if, therefore, we etideavor, 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 fur- 

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

 far as we can trace those effects which it would naturally leave behind 

 it on retiring ; and such the planets are. These suppositions being 

 made, it follows from known laws that successive zones of the solar 

 atmosphere would be abandoned ; that these would continue to revolve. 

 round the sun with the same velocity as when they formed part of his 

 substance ; and that they would cool down, long before the sun him- 

 self, to any given temperature, and consequently to that at which the 

 greater part of the vaporous rnatter 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 round its own axis, 

 a rotaXory movement ; and would in that state revolve, as the planets 

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

 tion, but with less velocity, and each of them in the same periodic time 

 which the sun's rotation occupied when his atmosphere extended to 

 that point; and this also M. Comte has, by the necessary calculations, 

 ascertained to be true within certain small limits of error.* There is, thus, 

 in Laplace's theory, nothing hypothetical : it is an example of legitimate 

 reasoning from a present effect to its past cause, according to the known 

 laws of that cause ; it assumes nothing more than that objects which 

 really exist, obey the laws which arc known to be obeyed by all ter- 

 restrial objects resembling them. The theory therefore is, as I have 

 said, of a similar character to the theories of geologists ; inferior to 

 them in certainty, in about the same ratio as those are inferior to facts 

 conclusively established by a judicial inquiry. For, the uncertainty 

 whether the laws, of nature which prevail on our earth prevail in the 

 whole solar system, is about equal to the uncertainty whether the laws 

 which prevail in our earth to-day prevailed there a thousand ages ago. 

 Laplace's theory requires both these assumptions, geology the latter 

 only, and judicial inquiries require neither.! 



♦ Cours de Pkilosophie Positive, ii., pp. 378-383. 



t See, for an interesting exi)dsition of this theory of Laplace, the Architecture of tht 

 Heavens, by Professor Nichol, of Glasgow ; a book professedly popular rather than scien- 

 tilic, but the production of a thinker who, both in this and in other departments, is capable 

 of much more than merely expounding the speculations of his predecessors. 



