8 
M. Hun)boldt on Isothermal Lines^ and the 
sagacity which distinguishes all his mathematical researches, has 
pointed out in his Pyrometrie the error of Mairan’s theory. 
He might have added, that this geometer confounds a quantity 
of heat which a point of the globe receives under the latitude of 
60® during the three months of summer, with the maximum to 
which the inhabitants of these northern regions see their 
thermometers rising in a clear day. The mean tempera- 
tures of the summers, far from decreasing from the pole to the 
tropics, are under the equator, under the parallel of 45®, and 
under that of Stockholm, Upsal, or St Petersburg, in the ratio 
of 81®. 86 ; 69®. 8 ; 61®. 16 of Fahrenheit’s scale. Reaumur had 
sent his new thermometers to the torrid zone, to Syria, and to the 
north. As it then reckoned sufficient to mark the warmest 
days, an idea was formed of an universal summer, which is the 
same in all parts of the globe. It had been remarked, and with 
reason, that the extreme heats are more frequent, and even more 
powerful, in the temperate zone in high latitudes, than under 
the torrid zone. Without attending* to the mean temperature 
of months, it was vaguely supposed, that in these northern re- 
gions the summers followed the ratio of the therm ometrical ex- 
tremes. This prejudice is still propagated in our own day, 
though it is well established, that in spite of the length of the 
days in the north, the mean temperatures of the warmest months 
at Petersburg, Paris, and the Equator, are 65®. 66; 69®. 44, and 
82®. 4. At Cairo, according to the observations of Nouet, the 
three months of summer are 84®. 74, and consequently 19® 
warmer than at St Petersburg, and 15® warmer than at Paris. 
The summer heats of Cairo, are almost equal to those I have 
experienced at Cumana and La Guayra betw een the tropics. 
With regard to the central emanation of the system of Mai- 
ran, or to the quantity of heat which the earth gives to the am- 
bient air, it is easy to conceive that it cannot act in all seasons. 
The temperature of the globe at the depths to which w e can 
reach, in general differs little from the mean annual temperature 
of the atmosphere. Its action is of great importance for the pre- 
servation of vegetables ; but it does not become sensible in the 
air, unless where the surface of the globe is not entirely covered 
with snow, and during those months, whose mean temperature 
