HABITIBILITY OF OTHER WORLDS. 135 



reduced on the earth to 2V its present amount, it is manifest that all organic life 

 must perish. If ever, therefore, the inherent temperature of Jupiter subsides so 

 far as to bring his surface condition to that of the earth, no Jovian climate will 

 be such as animal organization can endure. As his actual surface temperature, 

 however, will always be compounded of the effects of solar radiation and of con- 

 duction from within, there will be an epoch when his actual mean surface tem- 

 perature will be the same as the earth's actual mean surface temperature. The 

 vicissitudes of the seasons will be Jy as great as on the earth — regardless of the 

 effect of less obliquity of the axis — and the diurnal and nocturnal fluctuations of 

 temperature will be only ^V as great. Owing to a denser atmosphere, the fluctu- 

 ations will be even less than this. The higher inherent temperature of the soil 

 will result in so much radiation from the planet that on a planet with so large a 

 supply of water, and in an atmosphere so dense as Jupiter's the Sun's deficient 

 heat may be largely compensated by suppressed radiation from the planet. The 

 situation will be that of a mild and dimly lighted "stove " in horticultural opera- 

 tions, highly suitable for the growth of mushrooms. It will be perpetual even- 

 ing. It can not be doubted that corporeal intelligences might be coordinated to 

 such a physical condition. For the present, however, we have not the sHghtest 

 grounds for imagining the existence of organic populations upon the surface of 

 Jupiter, unless they depart in some very extreme way from the terrestrial stand- 

 ard. 



As to the planets remoter from the Sun, I have offered reasons for consider- 

 ing them advanced to a state of total refrigeration. They cannot therefore, be 

 conceived as habitable. There was a time, however, in the history of each, 

 when its stage of cooling produced a surface temperature suited for organic life. 

 At that stage, the relations of organic beings on their surfaces were similar to 

 those which may be anticipated for Jupiter, with all the greater divergences from 

 the terrestrial condition which depend on distance from the Sun carried to suc- 

 cessively greater extremes and successively larger proportions of water and gas- 

 eous substances. On Neptune the apparent diameter of the Sun is but -^-^ the 

 Sun's apparent diameter to us, and his heat and light are reduced to -g^-g- the 

 heat and light received by the earth. This light would, nevertheless, be equal 

 to about sixty-nine of our Moons. The excess of water however, on all the 

 distant planets, in accordance with views heretofore presented, would probably 

 render them, in all stages of existence, totally uninhabitable for beings like 

 ourselves. But it is always to be remembered that other beings suited to the 

 actual exigences of the environment, may have occupied the situation. 



The earth, then, so far as we can reason, is in the middle of the habitable 

 zone of the solar system, if our own natures are assumed as the criterion of habit- 

 ability. On either side, the rigor of the physical conditions seems to proclami 

 our system a voiceless and lifeless desert. Even our near neighbor, the Moon, 

 lies on the borders of this desert. Within the vast limits of the solar system there 

 is but one happy niche where corporeal organization according to our standard 

 can enter into material relations with the physical environment. The conclusion 



