HYPOTHESES. 27 



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 pro- 

 perty or law ascribed to a known substance. The 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 progressively cooling, and that, by the process of 

 cooling, it must contract ; if, therefore, we endeavour, 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 farther 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 retiring ; 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 j and that they would cool down, long 

 before the sun itself, to any given temperature, and conse- 

 quently 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 rotation, 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 in- 



