56 DENSITIES OF CELESTIAL BODIES. SECT. VIII. 



the sun is about 886,877 miles. Therefore, if the cen- 

 ter of the sUn were to coincide with the center of the 

 earth, his volume would not only include the orbit of 

 the moon, but would extend nearly as far again ; for 

 the moon's mean distance from the earth is about sixty 

 times the earth's mean radius, or 237,360 miles : so that 

 twice the distance of the moon is 474,720 miles, which 

 differs but little from the solar radius ; his equatorial 

 radius is probably not much less than the major axis of 

 the lunar orbit. The diameter of the moon is only 2160 

 miles ; and Jupiter's diameter of 87,000 miles is very 

 much less than that of the sun ; the diameter of Pallas 

 does not much exceed 79 miles, so that an inhabitant of 

 that planet, in one of our steam carriages, might go 

 round his world in a few hours. 



The densities of bodies are proportional to their 

 masses, divided by their volumes. Hence, if the sun 

 and planets be assumed to be spheres, their volumes 

 will be as the cubes of their diameters. Now, the ap- 

 parent diameters of the sun and earth, at their mean 

 distance, are 1922" and 17 //< 1552, and the mass of the 

 earth is the 354,936th part of that of the sun taken as 

 the unit. It follows, therefore, that the earth is nearly 

 four times as dense as the sun. But the sun is so large, 

 that his attractive force would cause bodies to fall 

 through about 334-65 feet in a second. Consequently, 

 if he were habitable by human beings, they would be 

 unable to move, since their weight would be thirty times 

 as great as it is here. A man of moderate size would 

 weigh about two tons at the surface of the sun ; where- 

 as at the surface of the four new planets he would be so 

 light, that it would be impossible to stand steady, since 

 he would only weigh a few pounds. The mean density 

 of the earth has been recently determined with a de- 

 gree of accuracy that leaves nothing farther to be de- 

 sired. Since a comparison of the action of two planets 

 upon a third gives the ratio of the masses of these two 

 planets, it is clear that if we can compare the effect of 

 the whole earth with the effect of any part of it, a com- 

 parison may be instituted between the mass of the 

 whole earth and the mass of that part of it. Now a 

 leaden ball was weighed against the earth by comparing 



