M. POISSON ON THE MATHEMATICAL THEORY OF HEAT. 137 



The mean of the annual temperatures, marked by a thermometer ex- 

 posed in the open £iir and in the shade, forms the climateric temperature. 

 It varies with the elevation of places above the level of the sea, and with 

 the longitude and latitude, according to unknown laws. At Paris it is 

 10°*822, as M. Bouvard has concluded after 29 years of observations. 

 There will be found in this Chapter a table of the mean temperatures for 

 the twelve months of each of those years, which that gentleman has been 

 pleased to communicate to us, and which had not before been published. 

 It appears that in every point of the earth this climateric temperature 

 differs very little from the mean temperature of the surface of the soil, 

 as is shown by several examples. Notwithstanding, the variable tem- 

 perature of this surface, and that which is marked at the same instant 

 by a thermometer as little elevated above the surface as may be, are 

 often very different from each other ; it hence follows, that in a year 

 the excess of the highest above the lowest temperature of the soil 

 is at Paris nearly 24°, as will be seen in the course of this Chapter; and 

 only about 17° for the thermometer suspended in the air and in the 

 shade. 



We now determine the part of exterior temperature which results from 

 the atmospherical heat combined with sidereal heat. The necessary data 

 for calculating its numerical value, a priori, being unknown to us, we 

 show how this value, for every point of the globe, may be deduced from 

 the mean temperature of its surface. At Paris this exterior temperature 

 is 13°. Although we cannot determine separately the portion of this 

 temperature of the earth which arises from the atmospherical lieat, there 

 is reason to think that it is also negative, so that the other portion arising 

 from sidereal heat must be less than 13° below zero. If we suppose that 

 radiant heat emanating from the stars falls in the same quantity on 

 all points of the globe, this temperature, higher than 13°, will be that 

 of space at the place where the earth is at this time. Without being 

 able to assign the degree of heat of space, we may however admit, 

 tliat its temperature differs little from zero, instead of being, as had 

 been asserted, below the temperature of the coldest regions in the 

 globe, and even of the freezing-point of mercury. As to the central 

 temperature of the whole mass of tiie earth, even supposing its ori- 

 ginal heat to be entirely dissipated, and that it is no longer equal to 

 the present tcmjicrature of space, we have no means of obtainino- a 

 knowledge of it. 



According to a theorem of Lambert, the whole amount of solar heat 

 which falls upon the earth is the same during different seasons, notwith- 

 standing the inequality of their lengtljs, which is found to be com- 

 pensated by that of the distances from the sun to the earth. This quan- 

 tity of licat varies in tlie inverse ratio of the parameter of the ellipse 

 described by tlie earth ; it also varies with the obliquity of the ecliptic. 



