SECT, vin.] DENSITIES OF CELESTIAL BODIES. 65 



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 apparent 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 ; whereas 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 degree of accuracy that leaves nothing farther to be desired. 

 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 comparison may be instituted be- 

 tween 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 the effects of each upon a pendulum ; the nearness 

 of the smaller mass making it produce a sensible effect as 

 compared with that of the larger : for by the laws of attrac- 

 tion the whole earth must be considered as collected in its 

 centre. By this method it has been found that the mean 

 density of the earth is 5'675 times greater than that of water 

 at the temperature of 62 of Fahrenheit's thermometer. The 

 late Mr. Baily, whose accuracy as an experimental philo- 

 sopher is acknowledged, was unremittingly occupied nearly 

 four years in accomplishing this very important object. All 



