so M. Humboldt on Isothermal Lines^ 
tern of climates in the northern and southern regions, we shall 
find, 
1^^, That the increase of the vernal temperature Is great, 
(from 14°. 4 or 16°.^, in the space of a month), and equally pro- 
longed, wherever the division of the annual heat between the 
seasons is very unequal, as in the north of Europe, and in the 
temperate part of the United States. 
That the vernal increase is great, (at least above 9° or 
10°.8), but little prolonged, in the temperate part of Europe. 
Sdly^ That the increase of the vernal temperature is small, 
(scarcely and equally prolonged, wherever there is an in- 
sular climate. 
Mhly^ That in every system of climates, in the zones contain- 
ed between the same meridians, the vernal increase is smaller, 
and less equally prolonged, in low than in high latitudes. 
The isothermal zone from 53°.6 to 55°. 4, may serve as an ex- 
ample for confirming these different modifications of spring. In 
Eastern Asia, near the concave summit, the differences of tem- 
perature between the four months of March, April, May and 
June, are very great, and very equal, (15°.7, 13°.8, and 13°.9). 
In advancing westward towards Europe, the isothermal line 
rises again, and in the interior of the country, near the convex 
summit, the increase is still greater, but little prolonged ; that is 
to say, that of the four months which succeed one another, there 
are only two whose difierence rises to 13° : they are 9°. 4; 13°. 3 ; 
4°.l. ^ Farther west, on the coasts, the differences become small 
and equal, viz. 3°.6; ‘6°.5; 5°.6. In crossing the Atlantic, we 
approach the western concave summit of the isothermal line of 
53°.6. The increase of vernal temperature shews itself anew, 
and almost as great, and as much prolonged, as near the Arctic 
concave summit. The differences of the four months are 10°.4 ; 
13°.9 ; and 10°.8. In the curve of annual temperature, the 
spring and autumn mark the transitions from the minimum and 
the maximum. The increments are naturally slower near the sum- 
mits than in the intermediate part of the curve. Here they are 
greater, and of longer continuance, in proportion to the diffe- 
rence of the extreme ordinates. The autumnal decrease of tem- 
perature is less rapid than the vernal increase, because the sur- 
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