324 Royal Society. 



like that of our evening clouds as to suggest the probability of its 

 being due to the deep atmospheric zone which has been often 

 ascribed to this planet, though perhaps, until of late years, on in- 

 sufficient grounds*. On this head spectral analysis will probably 

 enlighten us. If, however, there be such a deep covering of atmo- 

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

 wise appear unaccountable. Some considerable amount of vaporous 

 atmosphere 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, 

 HerschePs drawing for August 16, 1830f, with my sketch for Sep- 

 tember 27, 1862 X, 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 draw- 

 ing 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 geographical limit of the freezing mean winter tem- 

 perature, 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 within nearly the same thermic limits as those 

 of the earth. In all the broad belt of 30° or 40° from the equator, 

 the temperature seems to be such as always to allow of evaporation; 

 between that limit and the pole, snows gather and disperse according 

 to the season of the year, while for about 8° or 10° more or less 

 round the pole, the icy circle seems to be perennial. 



The relative mean distances from the Sun of Mars and the earth 

 being taken at 100 and 152, the relative solar influence must be 

 on Mars 100 to 231 on the earth; so that the surface of the more 

 distant planet might rather be expected to have shown signs of being 

 fixed in perpetual frost, than to have a genial temperature of 40° 

 to 50°, if not 50° to 60°, as the earth has, taken on the whole. 

 How is this to be accounted for ? Of two conceivable influences 

 which may be appealed to, viz. very high interior heat of the planet, 

 and some peculiarity of atmosphere, we may, while allowing some 

 value to each, without hesitation adopt the latter as the more 

 immediate and effective. 



To trace the effects in detail must be impracticable ; but in the 

 general we may remark that as a diminution of the mass of vaporous 



* " It has been surmised to have a very extensive atmosphere, but on no suffi- 

 cient or even plausible grounds." — Herschel's Astronomy (ed. 1833), p. 279, note, 

 f Treatise on Astronomy (ed. 1833), pi. 1. J Phil. Mag. S. 4. vol. xxvi. p. 314. 



