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



greater in high than in low latitudes, and in summer than in winter. If the thickness of 

 the stratum removed were greater the diminution of temperature would be greater in the 

 same ratio, as proved by Mr Welsh's observations to the height of 23,000 feet, and as may 

 be thence inferred, for much greater heights. An increase in the quantity of atmosphere 

 would produce an exactly similar but opposite effect on the temperature at the Earth's 

 surface. 



(3) After explaining Poisson's method of treating the problem of terrestrial 

 temperature, and his mode of estimating the direct effect of the solar heat radiating to the 

 Earth, on the temperature of its surface and of the atmosphere in contact with it, I have 

 shewn that the whole effect of the solar heat on the mean value of this superficial temperature 

 is equal to nearly twice the immediate effect which was the object of Poisson's calculation, 

 except in cases in which there may be a considerable horizontal transference of heat from 

 local causes. 



(4) Assuming that the temperature of every part of the Earth's surface would be 

 the same in the entire absence of the Sun's influence, I have estimated that temperature at 

 - S9°,5 (C). This must not be confounded with the temperature of that portion of stellar 

 space in which the solar system now exists, a temperature which we have not the means of 

 determining. 



(5) I have also shewn that annual inequalities in the temperature of the lower stratum 

 of the atmosphere are nearly equal in amount to those in the Earth itself at its surface, 

 with exceptions similar to those mentioned in (3). 



With these results we are prepared to estimate the external temperature of the planets 

 under certain assumed conditions. 



29. I commence with Jupiter. The intensity of solar radiation at the distance of 

 Jupiter, compared with that at the distance of the Earth is nearly as I : 27. Consequently 

 the value of the quantity which has been heretofore denoted by h will, in the case of Jupiter, 

 be only A-* of its value in the case of the Earth, so far as it depends only on the intensity of 

 radiation. Conceive the Earth itself to be placed at the same distance from the Sun as 

 Jupiter. If there were no sensible effect from solar radiation, the temperature of every 

 part of the surface would become the same, supposing equilibrium of internal and external 

 temperature to have become established in the same degree as it is in our globe at present. 



This temperature I have estimated at - 39°,5 (C) (Art. 25), assuming the intensity of 

 stellar radiation towards any point of the solar system to be the same in all directions. 

 To this temperature we must add that due to solar radiation at the distance of Jupiter. 

 The whole effect of this radiation, according to our calculation, would be nearly 2hQ; 

 and since hQ will, in the present case, be ^ th part of its actual value on the Earth, 

 we shall have 



2hQ = 2 i— = 2°,6 (C) 



-2 7i 



at the equator. Consequently, if the Earth were placed at the distance of Jupiter, the 

 mean, annual temperature at the equator would be - 36*°,q (C), or rather more than 20 (C) 



