PLANETARY MOTION. 31 



axes, in the same direction that the sun moves 

 upon his axis. 



3. That the heating power of the sun dimi- 

 nishes in proportion as the squares of the dis- 

 tance increase.* 



4. That the centrifugal force by which planets 

 and their satellites are impelled through their 

 orbits diminishes in proportion as the squares of 

 the distance from the centres of power increase. 



5. That the centripetal force by which planets 

 are impelled towards the sun, and satellites 



* For example, as Mercury is 1-888 times nearer to the sun 

 than Venus, he receives more caloric than Venus, in the ratio of 

 3-564 to one; because the square of 1-888 is 3-564. And as 

 Mercury is 2-638 times nearer to the sun than the earth, he 

 receives 6-959 times more caloric, or the square of 2-638. For 

 the same reason, as Mercury is 3-944 times nearer to the sun 

 than Mars, the latter receives 15-555 times less caloric: Ceres 

 25-157 times less; Jupiter, 185-259 times less on a given area; 

 Saturn 625 times less ; and Uranus 2500 times less. Or, if we 

 take our own planet as the standard of measure, the heating 

 power of the sun is 2-232 times greater at the earth than at Mars, 

 because he is 1-494 times farther from the sun; 7'371 times greater 

 than at Ceres; 26*594 times than at Jupiter ; 89-737 times than at 

 Saturn ; and 358-988 times greater than at Uranus. But as the 

 planets are impelled through their orbits by two equal and op- 

 posite forces that unite in support of each other, the velocities do 

 not diminish at the same rate as their mean distances from the 

 sun : for while Jupiter is 5*157 times farther from the sun than 

 the earth, his velocity is only about 2-30 times less. In like 

 manner, although Saturn is nearly 9J times farther from the sun 

 than the earth, his velocity is only about three times less ; and so 

 of all the other planets, as may be readily ascertained by com- 

 paring the times and distances in the following Tables, con- 

 structed from data furnished by Sir J. Herschel. (Treatise on 

 Astronomy.) 



