LE VERRIER AND ADAMS' PLANET. 



hold them in their orbits will be proportional to the squares of 

 these numbers. But the squares of these numbers, if computed, 

 will be found to be in the proportion of 2 1| to 1 very nearly. It 

 would follow, therefore, that the weight of Neptune is 21| times 

 that of the earth. 



But it must not be forgotten that in this calculation we have 

 supposed that the distance of the satellite from Neptune is equal 

 to the distance of the moon from the earth, but in fact the dis- 

 tance of Neptune's satellite is less than that of the moon, in the 

 proportion of 225 to 238, and, therefore, the estimate of the mass 

 of Neptune, computed as above, upon the supposition of the exact 

 equality of the two distances, must be reduced in the same pro- 

 portion, which would make Neptune's mass 20 times that of the 

 earth. 



This, as in the former case, is a very rough method of calcula- 

 tion, adopted to render familiar a problem, which, in its more 

 exact details, would be much too difficult for any but professed 

 astronomers perfectly to understand. By more exact methods it 

 appears that the weight of Neptune is about 19 times that of the 

 earth. 



23. The relative bulks, or volumes, of globes being in the pro- 

 portion of the cubes of their diameters, and the diameter of 

 Neptune being 37500 miles, while that of the earth is about 7900 

 miles, it follows that the bulk of Neptune is about 107 times 

 greater than that of the earth ; or that 107 globes like the earth, 

 being rolled into one, would form a planet equal to Neptune. 



24. Since the brightness of the sun's light, and the warmth 

 produced by its heat, decrease in the same proportion as the super- 

 ficial magnitude of his disc decreases, and since the diameter of 

 that disc decreases in the same proportion as the distance of the 

 observer from the sun increases, it follows that the superficial 

 magnitude of the sun's disc, and therefore the brightness of the 

 light of day, and the warmth of the solar rays, will be less at 

 Neptune than they are at the earth, in the same proportion 

 in which the square of Neptune's distance from the sun is greater 

 than the square of the earth's distance ; and since Neptune is 

 30 times more distant than the earth, it follows that the brightness 

 of day and the sun's warmth at Neptune are 900 times less than 

 at the earth, 



25. The apparent diameter of the sun, as seen from Neptune, 

 being 30 times less than from the earth, is, 



The sun, therefore, appears of the same magnitude as Venus 

 seen as a morning or evening star. 

 190 



