M. POISSON ON THE MATHEMATICAL THEORY OF HEAT. 137 
The mean of the annual temperatures, marked by a thermometer ex~ 
posed in the open air and in the shade, forms the elimateric temperature. 
It varies with the elevation of places above the level of the sea, and witlr 
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, @ 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 heat, 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, 
that 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 the earth, even supposing its ori- 
ginal heat to be entirely dissipated, and that it is no longer equal to 
the present temperature of space, we have no means of obtaining 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 lengths, which is found to be com- 
pensated by that of the distances from the sun tothe earth. This quan- 
tity of heat varies in the inverse ratio of the parameter of the ellipse 
described by the earth ; it also varies with the obliquity of the ecliptic. 
