536 PHILOSOPHICAL TRANSACTIONS. [ANNO 1784. 



58° 3l' preceding the intersection of the ecliptic with the orbit of Mars, or in 

 our 1 Q° 28' of Pisces. 



Being thus acquainted with what the inhabitants of Mars will call the obliquity 

 of their ecliptic, and the situation of their equinoctial and solstitial points, we 

 are furnished with the means of calculating the seasons on Mars ; and may 

 account, in a manner highly probable, for the remarkable appearances about its 

 polar regions. The analogy between Mars and the earth is perhaps by far the 

 greatest in the whole solar system. Their diurnal motion is nearly the same ; 

 the obliquity of their respective ecliptics, on which the seasons depend, 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 that which we enjoy, when compared to the surprizing 

 duration of the years of Jupiter, Saturn, and the Georgium Sidus. If then we 

 find that the globe we inhabit has its polar regions frozen and covered with 

 mountains of ice and snow, that only partly melt when alternately exposed to the 

 sun, we may well be permitted to surmise that the same causes may probably 

 have the same effects 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, 

 since that pole had but lately been involved in a whole twelve-months darkness 

 and absence of the sun ; but in 1783 it was considerably smaller than before, 

 and it decreased continually from the 20th of May till about the middle of Sept., 

 when it seemed to be at a stand. During this last period the south pole had 

 already been above 8 months enjoying the benefit of summer, and still continued 

 to receive the sun-beams; though, towards the latter end, in such an oblique 

 direction as to be but little benefited by them. On the other hand, in the year 

 1781, the north polar spot, which had then been its twelve-month in the sun- 

 shine, and was but lately returning to darkness, appeared small, though doubt- 

 less increasing 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; most probably, in the next opposition we shall see it 

 renewed, and of considerable extent and brightness ; as, by the position of the 

 axis of Mars, the sun's southern declination will then be no more than 0° 25' on 

 that planet. 



Of the spheroidical figure of Mars. — That a planetary globe, such as Mars, 

 turning on an axis, should be of a spheroidical form, will easily find admittance, 

 when two familiar instances in Jupiter and the Earth, as well as the known laws 

 of gravitation and centrifugal force of rotatory bodies, lead the way to the recep- 



