METEOROLOGY. 



325 



This point beinpj established, let us, in the next place, compare the 

 intensities received by the planets during entire revolutions in their 

 orbits ; and also the ratios of intensity for equal times, which depend 

 simply on the inverse square of the distance. The following table has 

 been tluis prepared from the usual astronomic elements. 



It should be observed that the foregoing table does not take account 

 of the different dimensions of the planets, but refers to a unit of plane 

 surface upon their disks, which is exposed perpendicularly to the rays 

 of the perpetual sun. Upon the disk of Mercury, the solar radiation 

 appears to be nearly seven times greater than on the earth ; while 

 upon Neptune, it is only as the one-thousandth part, in equal times. 

 In entire revolutions, however, the intensities received will be seen to 

 approach more nearly to equality. 



The intensities are thus unequal ; and, by a calculation founded on 

 the apparent brightness of the planets as estimated by the eye, Prof. 

 Gibbes has shown, in the Proceedings of the American Association 

 for the Advancement of Science for 1850, that the reflective powers 

 are also greater, according as the several planets are more distant from 

 the sun. 



Another feature worthy of mention, is the resemblance of the earth 

 to the planet Mars ; upon which Sir W. Herschel has remarked : 

 "The analogy between Mars and the earth is, perhaps, by far the 

 greatest in the whole solar system. The diurnal motion is nearly the 

 same, the obliquity of their respective ecliptics not very different ; of 

 all the superior planets, the distance of Mars from the sun is by far 

 the nearest alike to that of the earth ; nor will the length of the 

 Martial year appear very different from what we enjoy, when com- 

 pared to the surprising duration of the years of Jupiter, Saturn, and 

 Uranus. If we then find that the globe we inhabit has its polar 

 region frozen and covered with mountains of ice and snow, that only 

 partly melt when alternately exposed to the sun, I may well be per- 

 mitted to surmise that the same causes may have the same effect on 

 the globe of Mars ; that the bright polar spots are owing to the vivid 

 reflection of light from frozen regions ; and that the reduction of those 

 spots is to be ascribed to their being exposed to the sun." 



