DIURNAL REVOLUTION OF PLANETS. 43 



in the sun himself, and on which his radiating 

 power depends.* 



* The above views will also enable us to explain the diurnal 

 revolution of planets, the elliptical form of their orbits, the in- 

 clination of their axes, and all the phenomena of the tides. In 

 the first place, if the planets were perfectly round, and not in- 

 fluenced by each other, both the radiating power of the sun, and 

 the centripetal pressure of the aether, would be the same on all 

 parts of their surface, and they could have no rotary motion. Nor 

 could there be any elongations of their orbits, which would be 

 perfect circles, if the centrifugal and centripetal forces by which 

 they are impelled were always equal and the same. But as none 

 of the heavenly bodies are perfectly spherical, the solar radiation, 

 like the returning pressure of the sether, must operate with un- 

 equal force on different parts of their surface, and tend to give 

 them a rotary motion ; the rapidity of which may depend upon 

 the extent of their deviation from the perfectly spherical form, and, 

 perhaps, on the inclination of their axes : which, again arises from 

 their mutually disturbing influence upon each other, modified by 

 the difference between their equatorial and polar diameters. In 

 the case of Jupiter, this difference is as 107 to 100; whereas in 

 the earth, it is as 299 to 298. 



There is another circumstance which may have an equally im- 

 portant influence on the elliptical form of the planetary orbits. 

 For example, we learn from the telescopic observations of Sir 

 William Herschel, that all parts of the sun are not equally lumi- 

 nous ; that at times, there may be seen on his surface spots, some 

 of which are so large as to measure 45,000 miles in linear diame- 

 ter, exhibiting the appearance of a dark, opaque, and solid 

 ground. It therefore follows, that different quantities of caloric 

 are radiated from different parts of his surface, so as to augment 

 and diminish within certain limits, the centrifugal force of the 

 planets, causing them to approximate the sun in certain parts of 

 their orbits, and to recede from him in other parts, with corres- 

 ponding accelerations and retardations in their annual motions, 

 and, perhaps, variations in their mean annual temperature. On 

 the other hand, as the centripetal pressure of the eether on 



