I 
Distribution of Heat over the Globe. 1 
the results of calculation, not with the mean temperature drawn 
from observations made under different longitudes, but with 
that of a single point of the earth’s surface, we shall set out with 
that which is due to the immediate action of the sun, and to the 
whole of the other influences, whether they are solar or local, or 
propagated to great distances. This comparison of theory with 
experience will present a great number of interesting relations. 
In the year 1693, previous to the use of comparable thermo- 
meters, and to precise ideas of the mean temperature of a place, 
Halley laid the first foundations of a theory of the heating ac~ 
tion of the sun under different latitudes *. He proved that these 
actions might compensate for the effect of the obliquity of the 
rays. The ratios which he points out, do not 'express the mean 
heat of the seasons, but the heat of a summer day at the equa- 
tor and under the polar circle, which he finds to be as 1.834 to 
^.310 According to Geminus if, .Polybius among the Greeks 
had perceived the cause why there should be less heat at the 
equator than under the tropic. The idea also of a temperate 
zone, habitable and highly elevated in the midst of the torrid 
zone, was admitted by Esatosthenes, Polybius, and Strabo. 
In two memoirs §, published at long intervals in 1719 and 
1765, Mairan attempted to solve the problems of the solar action, 
by treating them in a much more extended and general manner. 
He compared, for the first time, the results of theory with those 
of observation ; and as he found the difference between the 
heat of summer and winter much less than it ought to be by 
calculation, he recognised the permanent heat of the globe and 
the effects of radiation. 
Without mistrusting the observations he employed, he con- 
ceived the strange theory of central emanations which increase 
the heat of the atmosphere from the equator to the pole. He 
supposes that these emanations decrease to the parallel of 74^, 
where the solar summers attain their maximum, and that they 
then increase from 74^ to the pole. Lambert^, with that 
* Phil. Tram'. 1693, p. 878. + This should be 2.339 . — ■. 
J Isag. in Aratum^ cap. 13. ; Strabo, Geogr. lib. ii. p. 97. 
§ Mon. (k VAcad. 1719, p. 133; and 1765, p. 145. and 210. 
■^[ Piircvieirh odcr Foin Maavc desi Feuos, 1779, p. 342. 
