662 Mr W. HOPKINS, ON THE EXTERNAL TEMPERATURE OF THE EARTH, 



unquestionable, and the necessary consequence would be that the addition of 35,000 or 40,000 

 feet to the thickness of her atmosphere would elevate her equatorial temperature to about the 

 mean temperature of our temperate zone at present. 



It is easy to pass from this hypothetical case of the Earth moving in Jupiter's orbit, 

 to the case of Jupiter himself, supposing him to have an atmosphere of the same nature as 

 to the transmission of heat as that of the Earth. The principal difference would be that 

 an atmosphere of given height would have a greater density about Jupiter than about the 

 Earth, on account of his greater attraction. But the very approximately uniform rate of 

 decrement of temperature in our actual atmosphere up to a height at which the density is less 

 than half that at the Earth's surface, implies that the transmissive power of the atmosphere is, 

 in a great degree, independent of its density; so that the greater density of Jupiter's 

 atmosphere, cceteris paribus, would probably have no material influence on the rate at which 

 the temperature would decrease in ascending from his surface. There seems reason to believe 

 that Jupiter has an atmosphere much greater than that of the Earth, and it may be of such 

 a thickness as entirely to counterbalance the defect of heat derived from the Sun by direct 

 radiation. His climate, however, must differ from that of the planets nearer to the Sun, 

 in the comparative absence of periodical variations, which can scarcely be sensible in his 

 equatorial regions, and can never amount to more than a few degrees in his polar regions. 



30. The cases of the planets more remote than Jupiter are still more simple than that of 

 Jupiter himself, since solar radiation can have no sensible effect whatever upon them. If the 

 Earth were placed in the position of any one of these planets, and equilibrium of temperatures 

 due to internal and external causes, respectively, were so far established as it is under existing 

 conditions, her superficial temperature would, according to our previous calculations, be about 

 — 39°, 5 (C) ; but if her atmosphere were increased to the requisite amount, this temperature 

 might be increased, as we have seen, in any proposed degree. And so it must be with these 

 remote planets themselves. Their superficial temperatures must be entirely uniform inde- 

 pendently of variations due to local or temporary causes, and must depend on the quantity 

 of atmosphere by which they may be respectively surrounded. 



31. It seems to be an opinion very generally received that a superficial temperature may 

 perhaps be preserved in these remoter planets, higher than that which may be due to external 

 causes, by the remains of the primitive heat which they may originally have possessed. This 

 notion, however, is contrary to all analogy with the case of our own globe ; for it is distinctly 

 proved that the superficial temperature of the Earth has already approximated within the very 

 small fraction of a degree to that ultimate temperature below which it can never descend so 

 long as its atmosphere and all other external conditions remain unaltered. This close ap- 

 proximation to its final state can be due only to the enormous length of time during which the 

 Earth must have been exposed to the cooling influences of surrounding space ; and these 

 influences must have been equally effective with respect to the other planets as with respect 

 to the Earth, supposing them to have existed for the same period of time, for the magnitudes 

 of the planets would have little effect on their superficial temperature after a sufficient lapse of 



