280 M. Humboldt on Isothermal Lmes^ 
leads us to an exact knowledge of the numerical ratios between 
the elevations and the latitudes, ratios which we find indicated 
wnth little precision in works on physical geography. 
The following are the results which I have obtained from exact 
data in the temperate zone j from the plains to 1000 metres of ele- 
vation. Every hundred metres of perpendicular height, dimi- 
nishes the mean temperature of the year, by the same quantity 
that a change of 1° of latitude does in advancing towards the 
Pole. If we compare only the mean temperature of summer, 
the first 1000 metres are equivalent to 0°.81 Fahr. From 40° 
to 50° of latitude, the mean heat of the plains in Europe de- 
creases in Europe 12°. 6 of Fahr.; and this same decrease of 
temperature takes place on the declivity of the Swiss Alps from 
0 to 1000 metres of elevation. 
Differences of Latitude, 
Compared with Differences of 
Elevation. 
Mean 
Heat 
of the 
Year. 
Mean 
Heat 
of 
Summer. 
Mean 
Heat 
of 
Autumn. 
I. At the Level of the Sea. 
a. Latitude, 40°i 
63J4 
77^00 
62!60 
b. Latitude, 50°, 
50.54 
64.40 
II. On the Declivity of Mountains, 
a. At the foot in 46° of Lat. 
53.60 
68.00 
51.80 
h. At an elevation of 1000 metres, 
41.00 
58.46 
42.80 
These numerical ratios are deduced from observations made 
on the temperature of the air. We cannot measure the quan- 
tity of heat produced by the solar rays on the parenchyma of 
plants, or in the interior of fruits which receive their colour in 
ripening. The fine experiment of MM. Gay-Lussac and The- 
nard, the combustion of chlorine and hydrogen, proves what a 
powerful action direct light exercises on the molecules of bodies. 
But as the extinction of light is less upon the mountains in dry 
and rarified air, maize, fruit-trees, and the vine, still flourish at 
heights whichj according to our thermometrical observations 
made in the air, and far from the ground, we ought to suppose 
leras, by comparing them to the temperature of certain months in France or iti 
Italy. We find in the plains of Orinoco the month of August of Rome ; at Po- 
payan, (2988 feet), the month of August of Paris ; at Quito (4894 feet), the month 
pf May ; in the Paramos, (5904 feet), the month of March at Paris. 
