-1859] WRITINGS OF JOSEPH HENRY. 149 



exposed to the clear sky, always falls by its own radiation 

 several degrees lower than the temperature of the air; which 

 it would not do if the temperature of space were not lower 

 than — 60°, since as it approached that temperature at places 

 near the pole, the extra cooling from exposure to the sky 

 would be very little. Mr. Esp}' concludes, from theoretical 

 data, that the estimate of Pouillet is near the truth. 



Pouillet finds, from the data given above, that the total 

 quantity of heat which space transmits in the course of a j^ear 

 to the earth and atmosphere, would be sufficient to melt a 

 stratum of ice upon our globe of 85'28 feet in thickness. 

 From other investigations of a similar character, which we 

 shall presently describe, he finds that the quantity of solar 

 heat received by the earth in the course of a year is sufficient 

 to melt 101 "68 feet of ice. From these two sources together 

 then the earth receives a quantity of heat sufficient to melt 

 187 feet of ice. These results are of so unexpected an amount 

 that though obtained by instruments and methods which 

 are apparently unexceptionable, they have not fully obtained 

 acceptance, and the subject is therefore still open for further 

 examination. 



Terrestrial temperature. — If the earth were exposed in space 

 without an envelope and without receiving radiation from 

 any source, it would sink to the zero of temperature, or that 

 at which the atoms would cease to vibrate, and this, accord- 

 ing to the mechanical theory of heat, would be about 500° 

 below the freezing point of Fahrenheit's scale. 



If the earth were exposed without an envelope to the tem- 

 perature of space it would, according to the results obtained 

 by Pouillet, fall to — 222° of the same scale. 



With the present envelope and stellar radiation it would 

 stand at — 128°. The heat necessary to make up the actual 

 temperature of the earth beyond this degree is due to the 

 sun's accumulated heat under the envelope. 



Pouillet has also made a series of researches on the abso- 

 lute amount of heat from the sun. He used in his inves- 

 tigations an instrument to which he gave the name of 

 pyr-heliometer (measurer of the heat of the sun). It con- 



