402 Cambridge Philosophical Society : — 



ternal heat which they probably possessed in the earlier stages of 

 their existence. It is a well-established conclusion, that the super- 

 ficial temperature of our own globe has arrived at that point below 

 which it can never descend by more than the small fraction of a 

 degree, so long as all external conditions remain the same as at 

 present ; and the superficial temperature of the remoter planets will 

 in all probability be reduced to the corresponding limit. To these 

 external conditions, therefore, and not to their {jrimitive heat, must 

 the existing temperatures on the surfaces of these planets be attri- 

 buted, assuming always that they are not of less antiquity than our 

 own globe. Hence the superficial temperature of the Earth, with its 

 present atmosphere, placed at the distance of Neptune, Uranus, or 

 Saturn, would be very nearly — 39°'5 C, since the effect of solar 

 radiation at those distances would be nearly insensible. But if the 

 extent of the atmosphere were increased, the superficial temperature 

 would be augmented in a corresponding degree. Judging by the 

 decrements of temperature observed by Mr. Welsh, the author con- 

 cludes that an increase in the height of the Earth's atmosphere of 

 35,000 or 40,000 feet, would elevate her superficial temperature, if 

 placed in the remoter planetary regions, to nearly the mean tempe- 

 rature of our present temperate zone. The same conclusion will 

 hold with respect to the three planets above mentioned, if we sup- 

 pose them to have atmospheres similar to that of the Earth, and of 

 sufficient extent. Their temperatures must be sensibly uniform over 

 the whole of their surfaces, not being subject to any appreciable 

 annual variation. 



The same conclusions will apply to Jupiter, except that there will 

 be a small augmentation of temperature arising from solar radiation, 

 which the author calculates might amount to about 2^° C. at his 

 equator. 



Hence the author concludes that those views which assign a 

 necessarily low temperature to the above-mentioned planets in con- 

 sequence of their distance from the Sun, are altogether untenable. 



The conditions under which Mars is placed approximate more 

 nearly to those of the Earth than for any other planet. The author 

 calculates, that with an atmosphere similar to that of the Earth, and 

 exceeding it in height by about 1.5,000 or 20,000 feet, the equatorial 

 temperature of Mars may be about 60° F., or 15°-5 C, and his polar 

 temperature about — 10°C. The extent of the annual variations 

 would be about half those on our own planet in corresponding lati- 

 tudes, supposing the conductivity, specific heat, and radiating power 

 of the matter composing his superficial crust to be the same as for 

 the Earth. 



Again, if the Earth, with her present atmosphere and obliquity, 

 were placed in the orbit of Venus, the mean equatorial temjierature 

 would be upwards of 90° C, subject to the reduction, which would 

 doubtless in this case be great, due to the horizontal transference of 

 heat. The mean polar temperature would be about 16° C. A 

 diminution in the atmosphere would reduce these temperatures in 

 any assigned degree. But the obliquity of Venus, though not satis- 

 factorily determined, is considered to be much greater than that of 



