1865.] Prof. Phillips on the Planet Mars. 45 



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 atmosphere 

 round the earth would greatly exaggerate the difference of daily and nightly, 

 and of winter and summer temperature, so the contrary effect would follow 

 from an augmentation of it. Applying this to Mars, we shall see that his 

 extensive atmosphere would reduce the range of summer and winter, and 

 of daily and nightly temperature. It would, moreover, augment the mean 

 temperature by the peculiar action of such an atmosphere, which, while 

 readily giving passage to the solar rays, would resist the return of dark heat- 

 rays from the terrestrial surface, and prevent their wasteful emission into 

 space *. This effect obtains now on the earth, which is rendered warmer, 

 as well as more equable in temperature, by the^ atmosphere than it would 

 be without it. It is conceivable that it may obtain upon Mars to a greater 

 degree, even without supposing the atmosphere to be materially different 

 in its nature from that round the earth, or the surface of Mars to have 

 any specially favourable or exceptional characters for the absorption and 

 radiation of heat. It seems, however, requisite to suppose a greater com- 

 munication of heat from the interior of the planet ; for otherwise the addi- 

 tional vapour, to which the warming effect is in the main to be ascribed, 

 could not probably be supported in the atmosphere. On the whole we 

 may, perhaps, be allowed to believe that Mars is habitable. 



Here, so far as direct observations upon the aspect of Mars are available, 

 we may pause. The researches of the lladcliffe Observer, lately in Oxford, 

 and formerly at Greenwich, have, however, brought into view a peculiarity 

 in the constitution of this planet which deserves special notice. Its figure 

 is spheroidal, as might be expected from the general laws of planetary 

 form ; but it is spheroidal in so high a degree as to be quite exceptional 

 in this respect. Computing by the known rotation- velocity, and the 

 admitted measures and mass of Mars, its ellipticity should be about ^L. 



* Life on the Earth, I860, p. 163-65. Tyndall's Researches, Proceedings of the Royal 

 Society, February 1861. 



