S70 
M. Humboldt on Isothermal Lines ^ 
part of the globe radiates in every direction, the interior of a 
spherical envelope which would rest on the top of the highest 
mountains, would receive the samo quantity of radiant heat as 
the lower strata of the atmosphere. The heat, it is true, will 
be spread over a surface a little greater ; but the difference of 
temperature will be insensible, since the radius of the spherical 
envelope will be to that of the earth as 1.001 to 1. 
Considering the earth as surrounded with an atmospherical 
fluid, it is obvious, that the air heated at its surface will ascend, 
dilate itself, and be cooled, either by dilatation, or, by a more 
free radiation across the other strata that are equally rarified. 
These are the ascending and descending currents, which keep 
up the decreasing temperature of the atmosphere 
The cold of mountains is the simultaneous effect, l^if. Of the 
greater or less vertical distance of the strata of air at the surface 
of the plains and of the ocean. Of the extinction of light, 
which diminishes with the density of the superincumbent strata 
of air *1* ; and, Sd, Of the emission of radiant heat, which is fa- 
voured by air very dry j, very cold, and very clear. The mean 
temperature of our present plains would be lowered, if the seas 
should experience a considerable diminution. The plains of 
continents would then become 'plateaux^ and the air which rest- 
ed on them would be cooled by the circumjacent strata of air, 
which, at the same level, would receive but a small portion of 
the heat emitted from the dry bottom of the seas. 
The following Table contains the results of observations 
which I have made near the Equator, on the Andes of Quito, 
and towards the northern extremity of the torrid zone, in the 
Cordilleras of Mexico. These results are true means, given ei- 
ther by stationary observations made during several years, or 
by insulated observations. In these last, we have taken into 
account the hour of the day, — the distance of the solstices, — the 
direction of the wind, — and the reflection from the plains. 
Essay on Heat and Moisture^ p. 11. ; and Geometry^ p. 495. 
•j- Humboldt on Refraction below 10°, Observ. Astron. tom. i. p. 126, 
Wells on Dew, p. 50. 
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