4 i2 THE WORLD MACHINE 



movements of those which have traversed it during the period 

 of its vast extent, ended by uniting them with the sun. It 

 will be seen, then, that only such comets now exist as lay out- 

 side of this area during this interval. As we can only observe 

 those which approach rather near to the sun in their perihelion, 

 their orbits must be very excentric. But at the same time it 

 is seen that their inclination must offer like irregularities as 

 if these bodies had been projected at hazard, since the solar 

 atmosphere in no wise influenced their movements. Thus, the 

 long period of the revolutions of the comets, the great excentricity 

 of their orbits and the variety of their inclinations, is explained 

 very naturally by means of this atmosphere. 



" But how has the latter determined the movements of 

 revolution and of rotation of the planets themselves ? If these 

 bodies had penetrated into this fluid mass (from the exterior), 

 its resistance would have caused them to fall into the sun. One 

 may surmise, therefore, that they have been formed out of 

 successive limits of this atmosphere, by the condensation of 

 zones which it has abandoned in the plane of its equator, in 

 the cooling and condensing of the surface of this star, as we have 

 seen in the preceding pages. One may conjecture further that 

 the satellites have been formed in a similar manner from the 

 atmosphere of the planets. The five phenomena already alluded 

 to (similarity of movement, slight excentricity, &c.) follows 

 naturally from this hypothesis, to which the rings of Saturn give 

 an added degree of probability. Finally, if in the zones succes- 

 sively abandoned by the solar atmosphere there were molecules 

 too volatile to unite among themselves, or to the celestial bodies, 

 these latter would, in continuing to circulate around the sun, 

 produce all the appearances of the zodiacal light, without opposing 

 any sensible resistance to the movement of the planets." 



To this slight sketch Laplace afterwards added some specula- 

 tions as to the mechanical cause of the rotation of this nebulous 

 mass, and the immediate cause of the splitting off of successive 

 zones. This heated mass would in cooling contract towards 

 the centre. As it contracted its velocity of rotation would, 

 in consequence of one of the laws of mechanics, constantly 

 increase. The time would come then when in the outer por- 

 tions of the mass the centrifugal force would exceed the 

 attractive power of the central portion. These outlying zones 



