9 
D'istribution of Heat over the Globe. 
is below that of the whole year. In the south of France, for 
example, the radiation of the earth may act upon the atmos- 
phere in the five months which precede the month of April. We 
speak here of the proper heat of the globe, of that which is in- 
variable at great depths, and not of the radiation of the surface 
of the globe, which takes place even at the summer solstice, and 
the nocturnal effects of which have furnished M. Prevost with 
an approximate measure of the direct action of the sun *. 
Mairan had found, that in the temperate zone the heat of the 
solar summer is to that of the solar winter as 16 to 1. M. Pre- 
vost admits for Geneva 7 to 1. Good observations have given 
me for the mean temperature of the summers and the winters at 
Geneva 34^.7 ; 64^.94 ; and at St Petersburg 46°.94 and 62°.06. 
These numbers neither express ratios nor absolute quantities, 
but thermometrical differences considei'ed as the total effect of 
the calorific influences ; the ratios fuimished by theory separate 
the solar heat from every other indirect effect. Euler was not 
more successful than Mairan in his theoretical essays on the so- 
lar heat. He supposes that the negative sines of the sun’s alti- 
tude during the night give the measure of the nocturnal cool- 
ing, and he obtains the extraordinary result •[*, that under the 
equator the cold at midnight ought to be more rigorous than 
during winter, under the poles. Fortunately, this great geo- 
meter attached but little importance to this result, and to the the- 
ory from which it is deduced. The second memoir of Mairan, 
without adding to the problems which had been attempted since 
the time of Halley, has at least the advantage of containing some 
general views on the real distribution of heat in different conti- 
nents. It is true, that the extreme temperatures are there con- 
stantly confounded with ttie mean temperatures ; but previous 
to the works of Cotte and Kirwan, it was the first attempt to 
group the facts, and to compare the most distant climates. 
Dissatisfied with the route followed by his predecessors, Lam- 
bert, in his Treatise on Pyrometry, directed his attention to two 
very different objects. He investigated analytical expressions 
for the curves, which express the variation of temperature in a 
Du Caloriqne rayonnant^ p, 271. 277. 292. *|- Comment. Petrop. tom. ii. p. 98. 
