S64 M. Humboldt on Isothermal Lines^ and the 
Roxburgh, the mean temperature of June is 89°.4 ; at Abu- 
sheer 93°.2, but at Cumana I have found it only 84°.6- 
With respect to the temperate zone, it has been long known, 
that from the parallel of the Canary Isles to the Polar Circle, 
the severity of the winter augments in a progression much more 
rapid than the summers diminish in heat. It is also known, 
that the climate of the islands and the coasts differs from that of 
the interior of continents, the former being characterised by 
mild winters and less temperate summers. But it is the heat 
of summer particularly which affects the formation of the amy- 
laceous and saccharine matter in fruits, and the choice of the plants 
that ought to be cultivated. As the principal object of this me- 
moir is to fix, -after good observations, the numerical relations 
between the unequal quantities of heat distributed over the 
globe, we shall now compare the mean temperatures of three 
months of winter and summer under different latitudes, and 
shew how the infections of the isothermal lines modify these 
relations. In following the curves of equal heat from west to 
east, from the Basin of the Mississippi to the eastern coasts of 
Asia, through an extent of 4000 leagues, we are struck with 
the great regularity which appears in the variations of the win- 
ter temperature. 
I. Differences of the Seasons from the Equator to the Polar 
Circle. 
Cisatlantic Region. 
Long. 1° W. and 1T° E. 
Transatlantic Region. 
Long. 58°— 72“ W. 
I sother- 
Mean Temperatui-e. 
Mean Temperature. 
mal 
Lines of 
68“ 
Winter. 
Summer. 
DifF., 
Winter. 
Summer. 
Diff. 
59°.0 
80°. 6 
21°.6 
53°.6 
80°. 6 
27°.0 
59 
44.6 
73.4 
28.8 
39.2 
78.8 
39.6 
50 
35.6 
68.0 
32.4 
30.2 
71.6 
41.4 
41 
24.8 
60.8 
36.0 
14.0 
66.2 
52.2 
32 
14.0 
53.6 
39.6 
1.4 
55.4 
54.0 
This table shews the increase of the difference between the 
winters and summers from 28° and 30° to | the parallels of 55f 
and 65°. The increase is more rapid in the Transatlantic 
Zone, where the isothermal lines of 32° and 50° approach one 
another very much ; but it is remarkable, that in the two zones 
which form the two systems of different climates, the division of 
