74 REPORT—1840. 
opacity, and cloudy weather; and even could this be accom- 
plished, we should yet want data for knowing what portion of 
the incident heat combines with the earth so as to affect the 
climate. M. Pouillet, we have seen, has estimated the total 
quantity of sunshine incident on our globe*, but this was irre- 
spective of modifications of weather, and of the variable quantity 
of sunshine in different latitudes. 
104. A practical @ posteriori determination of this most im- 
portant meteorological element of the total quantity of effective 
solar heat, affecting the thermometric mean of any climate, has 
been suggested by M. Poisson. It seems well established by 
observations in the Caves at Paris, and by observations on the 
temperature of the earth at Geneva, that the mean air-tempera- 
ture and mean superficial earth-temperature agree at these 
places, though their extremes differ both in amount and epoch. 
If, then, we can by any means find the ¢otal effect of the solar 
rays on the superficial earth stratum, we may assume that to 
be the effect due to the direct action of the sun in raising the 
temperature of the place. 
105. M. Poisson has accordingly attempted most ingeniously 
to connect the climateric effect of the solar rays with the indica- 
tions of thermometers sunk to small depths in the ground. From 
the value of the superficial range (whose logarithm is A in the 
expression of art. 93.) one of its factors 4 may be discovered, 
entering into combination with known or determinable quanti- 
ties, and this quantity A ist, for any given spot, a number pro- 
portional to the direct climateric effect of the sun’s rays which 
may be deduced from itt. Now let us admit the mathematical 
accuracy of this very intricate investigation, and the admissibility 
* Art. 78. t See art. 94, note. 
+ The value of A of art. 93, 94, is the following (in Poisson’s Notation, 
2bh : f 
page 497) : log D 37 sinwsiny — 2«Q), 
where a, y, #% are astronomical constants already mentioned. 
his the constant of art, 94. note. 
b is a constant depending on the superficial character of the soil, and also on 
its conductivity. 
D is a function of 6, of the specific heat of the soil, and of the longitude of the 
earth's perihelion. 
Q is a very complicated function of the astronomical constants which deter- 
mine the length of the day, and is one of a series of definite integrals of 
which the succeeding terms are neglected. 
By the combination of two observed values of A + B p (art. 93.) 6 and B are 
eliminated; the above expression contains only / and known quantities (Poisson, 
p. 499); and the product 4 Q expresses (p. 518) the number of degrees by 
which the annual mean temperature of the given place is affected by direct 
solar radiation. 
