SUPPLEMENTARY REPORT ON METEOROLOGY. 69 
inconnues, et l’on peut seulement supposer qu’elles sont peu 
considérables.’ We know, on the contrary, that they are so 
considerable, that, estimating the loss of radiant heat by a 
vertical passage through the atmosphere (7 6.) at only twenty- 
five per cent., at an angle of elevation of 25° the force of the 
solar rays would be reduced to ahalf, and at 5° to one-twentieth 
part. We know, indeed, that the difference of the direct effect 
of a vertical and a horizontal sun is due to this cause alone, exag- 
gerated, of course, immensely by the variable meteorological 
state of the atmosphere, which again is a function of the lati- 
tude. 
90. (2.) The receptive power of the surface is a datum which 
we find it very difficult directly to determine, and which, since 
the quantity of sunshine cannot (as we have seen} possibly be 
directly computed, must be inextricably mixed up with it. It 
might be a question, whether, by covering a tolerably extensive 
surface of soil, in which thermometers are inserted, with a com- 
position of known superficial conductivity, this element might 
not become known. 
91. (3.) The specific heat (c)and conductivity (4) of the soil are 
also inextricably mixed up together in the analysis; but either 
becoming known, the other may be inferred from thermometric 
observations carried below the surface. The specific heat seems 
that best adapted for laboratory experiments ; M. Elie de Beau- 
mont has assigned 0°5614 for the value of ¢ (that for an equal 
bulk of water being = 1)*, proper to the soil at the Observatory 
of Paris. 
92. To obtain the conductivity of the soila@ posteriort, it is for- 
tunately not necessary that the preceding theoretical estimation 
of the distribution of sunshine should be correct ; but there are 
other estimates into which it essentially enters, and which must 
therefore be received with corresponding caution. To facilitate 
reference to M. Poisson’s work, I will show how the simple and 
very satisfactory observation of maximum and minimum tempe- 
rature of the earth’s crust at given small depths (above the inva- 
riable stratum) may be made to yield a knowledge of some of 
the constants above referred to. 
93. Let the excess of annual maximum above annual minimum 
temperature at a depth p be expressed by A, 5 then 
log A, = A+ Bpt 
In which A of course denotes the log. range when p = 0 or 
* Poisson, Supplement, p. 4. 
+ M. Quetelet puts under this form M. Poisson’s equation.—See the memoir 
referred to below. 
