44 Prof. Phillips on the Planet Mars. [Jan. 26, 



to the line of sight, while in 1862 it was ohlique (about 26), we shall per- 

 ceive that though the snow about the south pole were really as extensive 

 in 1864 as it appeared to be in 1862, it could not possibly appear even 

 nearly so large, and in fact could barely be seen (as it was) under the 

 very small angle which it would subtend on the limb. There may, how- 

 ever, have been really less snow round the south pole, in consequence of 

 the longer action of the summer heat on Mars in 1864 than in 1862. 



The ruddv tint of the surface of the broad tracts of land is so constantly 

 observed in these parts as to claim to be regarded as characteristic of some 

 peculiarity in them some special kind of terrestrial substance for ex- 

 ample *. On the other hand, the tint is so much like that of our evening 

 clouds as to suggest the probability of its being due to the deep atmo- 

 spheric zone which has been often ascribed to this planet, though perhaps, 

 until of late years, on insufficient grounds f. On this head spectral analysis 

 will probably enlighten us. If, however, there be such a deep covering of 

 atmosphere, it might explain some facts regarding the climate which other- 

 wise appear unaccountable. Some considerable amount of vaporous atmo- 

 sphere there must be, to give origin to the beds of snow which alternately 

 invest and desert the opposite poles, if indeed either pole be ever quite free 

 from snow. 



In different Martial years the extent of the snow appears nearly the 

 same under nearly similar conditions. Compare, for instance, Herschel's 

 drawing for August 16, 1830 , with my sketch for September 27, 18G2 , 

 and that now presented for November 20, 1864. 



Snows appear to have been observed in mass as far from the south pole 

 as lat. 40. This occurred in April 1856, according to a drawing by Mr. 

 De la Rue : snow in lat. 50 or perhaps 45 North is the result of my 

 observations during this late opposition. Assuming this to be the geogra- 

 phical limit of the freezing mean winter temperature, we see at once that 

 it differs but little from that of the earth, on which the isothermal line of 

 32 varies, according to local peculiarities, from the latitude of 40 to that 

 of 60. If the snows on the land of Mars be compared with those on the 

 northern tracts of Asia and America, they will be found not to extend 

 further. And as the snows, if they do not actually disappear, are reduced 

 to small areas about either pole in its warm season, thus showing the mean 

 summer temperature there to be not less than 32, this confirms the 

 general impression that the variations of the climate of Mars are comprised 



* " In this planet we discern, with perfect distinctness, the outlines of what may be 

 continents and seas. Of these, the former are distinguished by that ruddy colour which 

 characterizes the light of this planet (which always appears red and fiery), and indicates, 

 no doubt, an ochrey tinge in the general soil, like what the red sandstone districts on the 

 earth may possibly offer to the inhabitants of Mars, 'only more decided. Contrasted 

 with this (by a general law in optics) the seas, as we may call them, appear greenish." 

 Herschel's Astronomy (ed. 1833), p. 279. 



t " It has been surmised to have a very extensive atmosphere, but on no sufficient or 

 even plausible grounds." Ibid. p. 279, note. 



t Treatise on Astronomy (ed. 1833), pi. 1. Proc. Roy. Soc. 1863. 



