182 WRITINGS OF JOSEPH HENRY, [1855- 



accelerated velocit}', and then gradually diminishes in in- 

 tensity until daylight, when the earth has arrived at the 

 minimum of temperature. After this, again, the earth begins 

 to receive more heat than it loses, and the temperature of 

 the air constantly rises again until 3 o'clock. If the earth 

 were to radiate heat as rapidly at night as it does in the 

 day the minimum temperature would be at about 9 o'clock 

 in the morning; but on account of the diminished radiation 

 with diminished temperature, the compensation takes place 

 about the rising of the sun. When the radiation towards 

 the sky is prevented by a transparent covering which admits 

 the radiation from the sun, as in the case of a house lighted 

 by windows in the roof, the maximum temperature takes 

 place at a much later period of the day; and indeed were 

 the radiation to the sky entirely stopped the temperature of 

 the earth would increase indefinitely. 



Temperatures below the surface. — At a certain depth below 

 the surface of the earth there is a stratum of invariable tem- 

 perature, the depth of which augments with the latitude, 

 and in our climate is from about 100 to 115 feet. In general 

 the temperature of this stratum appears to be a little more 

 elevated than the mean annual temperature of the surface, 

 and this excess appears to increase with the latitude. This 

 stratum, it is evident, cannot be a regular surface, since it 

 must necessarily partake in a considerable degree of the vary- 

 ing contour of the external surface of the earth. The first 

 observations which were made upon this subject were in the 

 cellars of the Observatory at Paris, at the depth of 67^ feet 

 below the surface. They extend over a period of more than 

 fifty years, and show an invariable temperature of 53°"28 F. 

 The thermometer used in these observations was a most 

 delicate one, constructed by Lavoisier, and it in no instance 

 showed a variation of one-tenth of a degree Fahrenheit above 

 or below 53°'2S; and even these variations, small as they 

 are have been traced to accidental causes. 



Below the surface of the ground, and at a depth of from 65 

 to 80 feet, but few observations have been made, and these 

 have been principally applicable to the middle latitudes of 



