4 PKOGEESS : ITS LAAV AND CAUSE. 



space which the nebulous mass still filled, and the unoccu* 

 pied space which it previously filled. There simultaneously 

 resulted a contrast in density and a contrast in tempera- 

 ture, between the interior and the exterior of this mass. 

 And at the same time there arose throughout it rotatory 

 movements, whose velocities varied according to their dis- 

 tances from its centre. These differentiations increased in 

 number and degree until there was evolved the organized 

 grouj) of sun, planets, and satellites, which we now know — 

 a group which presents numerous contrasts of structure 

 and action among its members. There are the immense 

 contrasts between the sun and planets, in bulk and in 

 weight ; as well as the subordinate contrasts between one 

 planet and another, and between the planets and their sat- 

 ellites. There is the similarly marked contrast between 

 the sun as almost stationary, and the planets as moving 

 round him with great velocity ; while there are the sec- 

 ondary contrasts between the velocities and periods of the 

 several planets, and between their simple revolutions and 

 the double ones of their satellites, which have to move 

 round their primaries while moving round the sun. There 

 is the yet further strong contrast between the sun and the 

 planets in resj^ect of temperature ; and there is reason to 

 suppose that the planets and satellites differ from each 

 other in their proper heat, as well as in the heat they re- 

 ceive from the sun. 



When we bear in mind that, in addition to these various 

 contrasts, the planets and satellites also differ in respect to 

 their distances from each other and their primary ; in respect 

 to the inclinations of their orbits, the inclinations of their 

 axes, their times of rotation on their axes, their specific grav- 

 ities, and their physical constitutions ; we see what a high 

 degree of heterogeneity the solar system exhibits, when 

 compared with the almost complete homogeneity of the 

 nebulous mass out of which it is supposed to have originated. 



