AND JF THE OTHER BODIES OF SPACE. 23 



masses thrown off from their primaries, exactly as the 

 primaries had previously been from the sun. The orbit 

 of any satellite is also to be regarded as marking the 

 bounds of the mass of the primary at the time when that 

 satellite was thrown off ; its speed likewise denotes the 

 rapidity of the rotatory motion of the primary at that par- 

 ticular juncture. For example, the outermost of the four 

 satellites of Jupiter revolves round his body at the distance 

 of 1,180,582 miles, showing that the planet was once 

 3,675,501 miles in circumference, instead of being, as now 

 only 89,170 miles in diameter. This large mass took ra- 

 ther more than sixteen days six hours and a half (the pre- 

 sent revolutionary period of the outermost satellites) to ro 

 tate on its axis. The innermost satellite must have been 

 formed when the planet was reduced to a circumference 

 of 309,075 miles, and rotated in about forty-two hours and 

 a half. 



From similar inferences, we find that the mass of the 

 earth, at a certain point of time, after it was thrown ofl 

 from the sun, was no less than 482,000 miles in diameter, 

 being sixty times what it has since shrunk to. At that 

 time the mass must have taken rather more than twenty- 

 nine and a half days to rotate, (being the revolutionary 

 period of the moon,) instead of, as now, rather less than 

 twenty-four hours. 



The time intervening between the formation of the 

 moon and the earth's diminution to its present size, was 

 probably one of those vast sums in which astronomy deals 

 so largely, but which the mind altogether fails to grasp. 



The observations made upon the surface of the moon 

 by telescopes tend strongly to support the hypothesis as 

 to all the bodies of space being composed of similar mat- 

 ters subject to certain variations. It does not appear that 

 our satellite is provided with that gaseous envelope which, 

 on earth, performs so many important functions. Neither, 

 is there any appearance of water upon the surface ; yet 

 that surface is, like that of our globe, marked by ine- 

 qualities and the appearance of volcanic operations. 

 These inequalities and volcanic operations are upon a 

 scale far greater than any which now exist upon the 

 earth's surface. Although, from the greater force of 

 gravitation upon its exterior, the mountains, other cir- 

 cumstances being equal, might have been expected to be 

 much smaller than ours, they are, in many instances, 



