S78 M. Humboldt on Isothermal Lincs^ 
plain, by an analogous cause, why the progressive cooling dimi- 
nishes between 1000 and 2000 metres. The great plateaus of 
the Cordilleras commence only at the height of 2600 or 2900 
metres, (8528 or 9512 feet); and I am of opinion, that the slow- 
ness with which the heat decreases in the stratum of air between 
1000 and 2000, is the triple effect of the extinction of light, or 
the absorption of the rays in the clouds, — of the formation of 
rain, — and the obstacle which the clouds oppose to the free 
passage of radiant heat. The bed of air of which we speak, is 
the region in which are suspended the large clouds which the 
inhabitants of the plains see above their heads. The decrease of 
temperature, which is very rapid from the plains to the region 
of clouds, becomes less rapid in that region ; and if this change 
is less sensible in the temperate zone, it is no doubt because at 
the same height, the effect of radiation there is less sensible than 
above the burning plains of the equinoctial zone. In these 
zones, too, the cooling appears to follow the same law in the 
beds of air of equal temperature ; but the force of radiation va- 
ries with the temperature of the radiating beds. 
The results which we have now discussed, deserve the prefe- 
rence over those which are deduced from observations made du- 
ring excursions to the tops of some lofty mountains. The first 
give for the 
Metres. Cent. Fahr. Metres. 
Equinoctial Zone^ 0 — 4900 1° or 1°.8 for 187* 
Temperate Zone, 0—2900 1° or I'.S for 174 
* This is the mean result or the measure of the distribution of heat in the 
whole column of air. The partial results are from the back of the Andes. 
Heights in 
Cent. 
Fahr. 
Metres. 
Metres. 
0—1000 
or 
IS 
for 
170 
1000—2000 
1 
or 
1.8 
for 
294 
2000—3000 
1 
or 
1.8 
for 
232 
3000—4000 
1 
or 
1.8 
for 
131 
4000—5000 
1 
or 
1.8 
for 
180 
In these numbers, we recognise, as in the above Table, the influence of the region 
of clouds upon the decrease of heat. In order to shew the utility of these numeri- 
cal ratios, 1 shall give here the approximate calculation of the height of the plain 
of Thibet, deduced from the mean temperature of the month of October, which, ac- 
cording to the former, is 42°.26. As the latitude of Tissoolumbo 29®, gives 69°.8 
for the mean temperature of the plain ; and as at Mount St Gothard, the mean 
