TERRESTRIAL HEAT. 



suspended in space, all other bodies from which heat could be 

 supplied to it being removed, the heat which now pervades tlio 

 earth and its surrounding atmosphere would be necessarily dis- 

 sipated by radiation, and would thus escape into the infinite 

 depths of space. The temperature of the atmosphere, and those 

 of the successive strata, extending from the surface to the centre of 

 the globe, would thus be continually and indefinitely diminished. 



As no such fall of temperature takes place, and as, on the con- 

 trary, the mean temperature of the globe is maintained at an 

 invariable standard, the variations incidental to season and climate 

 being all periodical, and producing in their ultimate result a 

 mutual compensation, it remains to be shown from what sources 

 the heat is derived which maintains the mean temperature of 

 the globe at this invariable standard, notwithstanding the large 

 amount of heat which it loses by radiation into the surrounding 

 space. 



All the bodies of the material universe, which are distributed 

 in countless numbers throughout the infinitude of space, are sources 

 of heat, and centres from which that physical agent is radiated in 

 all directions. The effect produced by the radiation of each of 

 these diminishes in the same proportion as the square of its 

 distance increases. The fixed stars are bodies analogous to our 

 sun, and at distances so enormous that the effect of the radiation 

 of any individual star is altogether insensible. When, however, 

 it is considered that the multitude of these stars spread over the 

 firmament is so prodigious that in some places many thousand 

 are crowded together within a space no greater than that occupied 

 by the disc of the full moon, it will not be matter of surprise 

 that the feebleness of thermal influence, due to their immense 

 distances, is compensated to a great extent by their countless 

 number; and that, consequently, their calorific effects in those 

 regions of space through which the earth passes in its annual 

 course is, as will presently appear, not only far from being insen- 

 sible, but is very little inferior to the calorific power of the sun 

 itself. 



"We are, then, to consider the waste of heat which the earth 

 suffers by radiation as repaired by the heat which it receives from 

 two sources, the sun and the stellar universe ; and it remains to 

 explain what is the actual quantity of heat thus supplied to the 

 earth, and what proportion of it is due to each of these causes. 



48. An elaborate series of experiments were made by M. Pouillet, 

 and concluded in 1838, with the view of obtaining, by means 

 independent of all hypothesis as to the physical character of the 

 sun, an estimate of the actual calorific power of that luminary. 

 A detailed report of these observations and experiments, and an 

 82 



