338 M. Humboldt on the Difference of the 



derable to enable Us, from the unequal increase of temperature 

 which has been hitherto observed in them, to give the satisfac- 

 tory numerical solution of a problem which occupies the cu^ 

 riosity of men who live, as it were, upon a vault of rocks. Suf- 

 fice it here to point out how the recent views of geologists have 

 revived the old mythus of Pyroplegeton and of Hephastos. 



When a planet is everywhere surrounded by aerial strata, 

 and when the oxidised surface of the, earth, with its clefts 

 almost everywhere closed or filled up, by a long radiation 

 of heat, has arrived at a state of equilibrium between receiving 

 and losing, in such a manner that its external temperature and 

 the difference of climates arise solely from its position towards 

 the sun, towards a larger central body which is perpetually ge- 

 nerating light, then the problem of the temperature of any place 

 in its most general form, may be considered as dependent solely 

 upon the manner in which the influence of the meridian height 

 of the sun manifests itself. This height determines, at the same 

 time, the magnitude of the semidiurnal circles, the density of 

 the aerial strata, through which the rays of the sun pass, before 

 they arrive at the horizon ; it also determines the quantity of the 

 absorbed or calorific rays (a quantity which rapidly increases 

 with the size of the angle of incidence) ; and, lastly, the number 

 of the rays of the sun, which^ mathematically considered, a given 

 horizon receives. The production of heat, as far as a greater or 

 less is concerned, can accordingly be considered as proceeding 

 from the illuminated surface of the earth. The absorption 

 which the rays of the sun undergo in their passage through the 

 atmosphere, or (to express it in another manner) the production 

 of heat by the diminution of light is extremely small ; but never- 

 theless is perceptible on the ocean, where, at a great distance 

 from the coast, and even whfen the water was colder than the 

 atmosphere, I observed the temperature of the latter increasing at 

 noon with the height of the sun *. 



Recent researches + have shewn, that, in both continents, 



• Mr Arago has first called my attention to this remarkable effect of the 

 absorption of light in the atmosphere—Con. des Terns pour 1828, p. 225. 



f Essai Politique sur I'lsle de Cuba, 1826, t. ii. p. 79-92. where I think 

 I have obviated the doubts raised by Mr Atkinson.— Mem. of the Astron. 

 Spc vol. ii. p. 137, 137- 



