TERRESTRIAL HEAT. 



It is evident from the uniform calorific effects produced by the 

 solar rays at the earth, while the sun revolves on its axis exposing 

 successively every side to the earth in the course of about twenty- 

 five days, that the calorific emanation from all parts of the solar 

 surface is the same. Assuming this, then, it will follow, that the 

 heat which the surface of a sphere surrounding the sun at the 

 distance of the earth would receive would be so many times more 

 than the heat received by the earth as the entire surface of such 

 sphere would be greater than that part of it which the earth 

 would occupy. The calculation of this is a simple problem of 

 elementary geometry. 



But such a spherical surface surrounding the sun and con- 

 centrical with it, would necessarily receive all the heat radiated 

 by that luminary, and cue result of the calculation proves that the 

 quantity of heat emitted by the sun per minute is such as would 

 suffice to dissolve a shell of ice enveloping the sun, and having a 

 thickness of 38^ feet ; and that the heat emitted per day would 

 dissolve such a shell, having a thickness of 55,748 feet, or about 

 10| miles. 



50. The most powerful blast furnaces do not emit for a given 

 extent of fire surface more than the seventh part of this quantity 

 of heat. It must therefore be inferred that each square foot of the 

 surface of the sun emits about seven times as much heat as is issued 

 by a square foot of the fire surface of the fiercest blast furnace. 



51. When the surface of the earth during the night is exposed to 

 an unclouded sky, an interchange of heat takes place by radiation. 

 It radiates a certain part of the heat which pervades it, and it 

 receives, on the other hand, the heat radiated from two sources, 

 1 st, from the strata of atmosphere, extending from the surface of 

 the earth to the summit of the atmospheric column ; and 2nd, 

 from the celestial spaces, which lie outside this limit, and which 

 receive their heat from the radiation of the countless numbers of 

 suns which compose the stellar universe. M. Pouillet, by a series of 

 ingeniously contrived experiments and observations, made with the 

 aid of an apparatus contrived by him, called an actinometer, 

 has been enabled to obtain an approximate estimate of the pro- 

 portion of the heat received by the earth which is due to each of 

 these two sources, and thereby to determine the actual temperature 

 of the region of space through which the earth and planets move. 

 The objects and limits of this work do not permit us to give the 

 details of these researches, and we must therefore confine ourselves 

 here to the statement of their results. 



It appears from the observations, that the actual temperature of 

 space is included between the minor limit of 315 and the major 

 limit of 207 below the temperature of melting ice, or between 

 84 



