Besides these dark spots, former as tronomers took notice that a segment of his globe about the South pole exceeded the rest of his disk so much in brightness, that it appeared beyond them as if it were the segment of a larger globe. Maraldi informs us, that this bright spot had been taken notice of for sixty years, and was more permanent than the other spots on the planet. One part of it is brighter than the rest, and the least bright part is sub ject to great changes, and has sometimes disappeared.
A similar brightness about the North pole of Mars was also sometimes ob served ; and these observations are now confirmed by Dr. Herschel, who has viewed the planet with much better in struments, and much higher magnifying powers, than any other astronomer ever was in possession of His observations were made with a view to determine the figure of the planet, the position of his axis, &c. See Philosophipal Transactions, vol. lxxiv.
" The analogy," says Dr. Herschel, "between Mars and the earth, is, perhaps, by far the greatest in the whole solar sys tem. Their 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 compared to the surprising duration of the years of Jupiter, Saturn, and the Herschel. If then we 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 permitted to surmise, that the same causes may proba bly 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. In the year 1781, the South polar spot was extremely large, which we might well expect, as that pole had but lately been involved in a whole twelvemonth's darkness and absence of the sun ; but in 1783, I found it consi derably smaller than before, and it de creased continually from the20th of May till about the middle of September, when it seemed to be at a stand. During this last period the South pole had already been above eight months enjoying the benefit of summer, and still continued to receive the sun-beams, though, towards the latter end, in such an oblique direc tion, as to be but little benefited by them. On the other hand, in the year 1781, the North polar spot, which had been its twelvemonth in the sun-shine, and was but lately returning into darkness, ap peared small, though undoubtedly • creasing in size. Its not being visible in
the year 1783, is no objection to these phenomena, being owing to the position of the axis, by which it was removed out of sight. It has been commonly related by astronomers, that the atmosphere of this planet is possessed of such strong re fractive powers, as to render the small Axed stars near which it passes invisible. lir. Smith relates an observation of Cassini, where a star in the water of Aquarius, at the distance of six minutes from the disk of Mars, became so faint before its occultation, that it could not be seen by the naked eye, nor with a three feet tele scope. This would indicate an atmos phere of a very extraordinary size and density ; but the following observations of Hr. Herschel seem to show that it is of much smaller dimensions. "1783, Oct. 26th. There are two small stars pre ceding Mars, of different sizes ; with 460 they appear both dusky red, and are pretty unequal ; with 218 they appear considerably unequal. The distance from Mars of the nearest, which is also the largest, with 227 measured 3' 26" 20"'. Some time after, the same evening, the distance was 3' 8" 55"', Mars being retro grade. Both of them were seen very distinctly. Tpey were viewed with a new twenty feet reflector, and appeared very bright. October 27th, the small star is not quite so bright, in proportion to the large one, as it was last night, being a good deal nearer to Mars, which is now on the side of the small star ; but when the planet was drawn aside, or out of view, it appeared as plainly as usual. The distance of the small star was 2' 5" 25"'. The largest of the two stars (adds lie,) on which the above observations were made, cannot exceed the twelfth, and the smallest the thirteenth or tburteenth mag nitude ; and I have no reason to suppose that they were any otherwise affected by the approach of Mars, than what the brightness of its superior light may ac count for. From other phenomena it ap pears, however, that this planet is not without a considerable atmosphere ; for besides the permanent spots on its face, I have often noticed occasional changes of partial bright belts, and also once a darkish one in a pretty high lati tude ; and these alterations we can hardly ascribe to any other cause than the variable disposition of clouds and va pours floating in the atmosphere of the planet."