10 M. Humboldt 07i Isothei'inal Lines ^ and the 
place where it had been observed, and he resumed in its great- 
est generality the theorem of solar action. He gives formulae, 
from which we may find the heat of any day at all latitudes ; 
but being perplexed with the determination of the nocturnal dis- 
persion of the acquired heat, or the subtangents of the noctur- 
nal cooling *5 he gives tables of the distribution of heat under 
different parallels, and in different seasons i*, which deviate so 
much from observation, that it would be very difficult to ascribe 
these deviations to the heat radiating from the globe, and to dis- 
turbing causes. We are struck with the slight difference which 
the theory indicates between the mean annual temperatures of 
places situated under the equator and the polar circle, and be- 
tween the summers of the torrid zone and those of the tempe- 
rate zone. It cannot be expected, indeed, that analysis is ca- 
pable of determining the distribution of heat such as it exists 
on the surface of the globe. Without employing empirical 
laws, and deducing the data from actqal observation, the theo- 
ry can subject to calculation only a part of the total effect, 
or that which belongs to the immediate action of the solar rays ; 
but after the recent successful applications of analysis to the 
phenomena of the radiation of surfaces, the transmission of heat 
through solid bodies, and the cooling of these bodies in media 
of variable density, we may still expect to be able to perfect 
the theory of solar action, and to compute the distribution of 
the heat received into the exterior crust of our planet. 
In discussing what may be expected from the purely theoretical 
labours of Geometers, I have not spoken of a celebrated, but 
very concise Memoir of Mayer, the reformer of the Lunar 
Tables. This work, written in 1755, was published twenty 
years afterwards, in his Opera Inedita ;{;. It is a method, and 
* Pyromttrie^ p. 141. 179. Id. 318. 339. 
:J; De Variationibvs Thermomttri accuratius definiendis., ( Opera inedita.) vol. i. 
p. 3 — 10.) M. Daubuisson, in a note inserted in the Journal de Fhysiqve.) tom. Ixii. 
p. 449. has given a formula which accords better with observation than that 
of Mayer. He admits that the temperature increases from the pole to the equa- 
tor, as the cosine of the latitude raised to the power of 24° ; but he judicious- 
ly adds, that this formula is applicable only to a zone of the Old World, near 
the Northern Atlantic Ocean. — H. 
