Mr. Hopkius on the External Tempe^-ature of the Earth. 401 



the superficial temperature of the planet. In the Earth's atmosphere, 

 we are sure (as just remarked) that the indications of a thermometer 

 would constantly increase in its descent from a very high point above 

 the Earth's surface ; and therefore it follows, that if a planet be en- 

 veloped in an atmosphere similar to that of the Earth, but of o-reater 

 height, the superficial temperature of that planet will be higher than 

 that of the Earth, supposing both to exist in the planetary space un- 

 affected by the heat which radiates from the Sun ; while, on the 

 contrary, the superficial temperature of the planet would necessarily 

 be less, under the same conditions, than that of the Earth, if its 

 atmosphere were smaller, unless it should be so small as not to allow 

 a thermometer descending in it to reach its minimum indication. If 

 the planet were entirely without atmosphere, its superficial tempe- 

 rature (in the assumed absence of solar radiation) would be that of 

 surrounding space ; but we have no means of determining what rela- 

 tion that temperature bears to existing terrestrial temperature, or to 

 what this latter temperature would become in the absence of solar 

 radiation. 



The author has calculated from Poisson's formulae the increase of 

 temperature in the superficial crust of the Earth, due to the amount 

 of heat received by direct radiation from the Sun, in different lati- 

 tudes, above that temperature which would be common to all parts 

 of the Earth's surface in the absence of solar radiation, and with a 

 uniformity of intensity of stellar radiation in all directions upon our 

 globe. But this increased temperature must produce an augmentation 

 of temperature in the atmosphere, which must react on the terrestrial 

 temperature till equilibrium of temperature be established. The 

 author has endeavoured to estimate the amount of this indirect effect 

 of solar radiation by means of the data furnished by M. Dove's work 

 on terrestrial temperatures, combined with calculations based on 

 Poisson's formulae. He concludes that the whole effect of solar heat 

 at any proposed place is very nearly double that due to the im- 

 mediate and direct effect of solar radiation. Having thus ascertained 

 this entire effect, he finds the temperature which would pervade the 

 whole surface of the earth if the solar heat were extinguished. He 

 estimates this temperature at — 39°-.5 C. 



The annual variation of temperature in any latitude is found to be 

 nearly the same in amount for the terrestrial surface and for the part 

 of the atmosphere resting uj)on it. This must be understood as 

 applying to those places at which the temperature is not materially 

 affected by the horizontal transference of heat by marine or aerial 

 currents, or any local causes, which disturb the dependence of tern- 

 perature on latitude alone. The author also jjoints out the depend- 

 ence of the annual inequalities of the terrestrial temperature (and 

 consequently of those also of the atmosphere) on the conductivity 

 and specific heat of the matter which constitutes the Earth's crust. 

 If these were much greater, the annual changes of temperature would 

 be much less. 



Before apj)lying these results to other planets, the author states 

 that he does not adnit the notion, that the remoter planets may derive 

 a considerable superficial temperature from the remains of that in- 



