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



The annual inequality at the pole would doubtless be subject to a very considerable 

 reduction in an atmosphere resembling the Earth's, on account of the rapid degradation of 

 temperature in passing from the pole to the equator, at the time when this inequality should 

 be near the above, i. e. its greatest value, and when also the semi-annual inequality at the 

 equator would reduce the temperature there to nearly its lowest value. Judging by the 

 tendency to equalization of temperature in our own tropical regions, it seems probable that the 

 temperature would be lowered at the pole of Venus by many degrees, while that at her 

 equator would be increased. It must also be recollected that, as already explained, both 

 these inequalities in the temperature might be lessened in any degree by a change in the value 



of the quantity — . As the atmosphere would appear to be the principal agent with which 



we are acquainted for regulating the mean temperatures of the planets, so the conductivity, 

 specific heat of their outer crusts, and transmissive powers of their surfaces, appear to afford 

 the most effective agency by which we can understand the greatest modifications to be made in 

 the periodical variations of temperature to which the nearer planets may be subjected. The 

 densities of Jupiter and Saturn, combined with the enormous pressure to which their central 

 portions must be subjected, shew that the matter of which those planets are composed must 

 be of different kinds, while each must differ still more from the Earth ; and though the densi- 

 ties of Venus and Mars do not afford similar indications of difference, it would at least seem 

 contrary to the analogy afforded by the larger planets to assume the matter of which the two 

 former are constituted, to have exactly the same properties as the crust of our own globe. 



To take a numerical illustration, suppose that the tendency to equalize the temperature of 

 the atmosphere at the poles should reduce the coefficient of the annual inequality there from 

 75° (C) to 60°. Also suppose the matter composing the superficial crust of Venus to be such 



clc 

 that the fraction — should be five or six times as great as for the Earth, which is entirely 

 p i 



admissible as a supposition, considering the differences in the values of c, k, and p, which we 

 find in different terrestrial substances. This would reduce the coefficient of the inequality by 

 nearly 20°. Thus the reduction of the coefficient by both these causes would amount to nearly 

 35°, leaving it equal to rather more than 40° (C); and if we take the value of the mean tempera- 

 ture as above estimated (about 40°) with the diminished atmosphere, the extreme temperature 

 on the surface of Venus (which would be at the poles) would not much exceed 80° (C). At 

 the same time, taking the mean temperature of the equator at 12° (C) and the coefficient of 

 the semi-annual inequality as modified by the causes above mentioned, at 15°, the equatorial 

 temperature would oscillate between — 3° and + 27° (C). If we further suppose the sunshine 

 to be modified as above intimated, a large portion of the surface of Venus would be habitable 

 by beings little better calculated than the human race to bear the extremities of heat and cold. 

 If we should take a case intermediate, as regards the obliquity, we should obtain inter- 

 mediate results. The mean temperatures at the equator and pole would be more nearly equal, 

 and the oscillations of temperature due to the annual and semi-annual inequalities would 

 diminish with the obliquity. The general high temperature of this planet would in such case be 

 more equably diffused than in either of those cases which we have here examined in more detail. 



