PROGRESSIVE EFFECTS. 



333 



process of cooling, it must contract ; 

 if, therefore, we endeavour, from the 

 .present state of that luminary, to in- 

 fer 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 sup- 

 positions being made, it follows from 

 known laws that successive zones of 

 the solar atmosphere might be aban- 

 doned ; 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 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 gravita- 

 tion would then cause them to ag- 

 glomerate 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 La- 

 place's theory, nothing, strictly speak- 

 ing, hypothetical ; it is an example 

 of legitimate reasoning from a pre- 

 sent 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 con- 

 siderably inferior to them in point 

 of evidence. Even if it were proved 

 (which it is not) that the conditions 

 necessary for determining the breaking 

 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 whfcl) existed at the origin 



of the solar system, than in merely 

 presuming (with geologists) that those 

 laws have lasted through a few revo- 

 lutions and transformations of a single 

 one among the bodies of which that 

 system is composed. 



CHAPTER XV. 



OF PROGRESSIVE EFFECTS ; AND OF THE 

 CONTINUED ACTION OF CAUSES. 



§ I. In the last four chapters we 

 have traced the general outlines of 

 the theory of the generation of deri- 

 vative laws from ultimate ones. In 

 the present chapter our attention will 

 be directed to a particular case of the 

 derivation of laws from other laws, 

 but a case so general, and so impor- 

 tant, as not only to repay, but to re- 

 quire, a separate examination. This 

 is the case of a complex phenome- 

 non, resulting from one simple law, by 

 the continual addition of an effect to 

 itself. 



There are some phenomena, some 

 bodily sensations, for example, which 

 are essentially instantaneous, and 

 whose existence can only be pro- 

 longed by the prolongation of the 

 existence of the cause by which they 

 are produced. But most phenomena 

 are in their own nature permanent; 

 having begun to exist, they would 

 exist for ever unless some cause in- 

 tervened having a tendency to alter 

 or destroy them. Such, for example, 

 are all the facts or phenomena which 

 we call bodies. Water, once pro- 

 duced, will not of itself relapse into 

 a state of hydrogen and oxygen ; such 

 a change requires some agent having 

 the power of decomposing the com- 

 pound. Such, again, are the positions 

 in space and the movements of 

 bodies. No object at rest alters its 

 position without the intervention of 

 some conditions extraneous to itself ; 

 and when once in motion, no object 

 returns to a state of rest, or alters 

 either its direction or its velocity, 

 unless some pew external conditions 



