and the Distribution of Heat over the Globe. 27^ 
cular climates, modified by the radiation of the plateaus on 
which they stand, -^upon the slope of the ground, — the naked- 
ness of the soil, — the humidity of the forests, — -and the currents 
which descend from the neighbouring summits. 
Without knowing the localities themselves, the effect of dis- 
turbing causes will be readily seen, by comparing in the pre- 
ceding Table the mean temperatures which correspond to the 
same elevations ; and the discussion of these observations would 
prove, also, that the extent of the variations is much less than is 
generally believed. If we examine thirty-tw'o temperatures, upon 
the hypothesis that a degree of cooling corresponds to an altitude 
of 200 metres (656 feet), we shall deduce the temperature of the 
plains (from 30°. 6 to 82°. 4) twenty-six times from that of elevated 
places. For the other six deductions, the temperatures differ on- 
ly about 3°,6 ; and the errors of observation are here combined 
with the effects of localities. The air which rests on the plains of 
the Andes mixes itself with the great mass of the free atmosphere, 
in which there prevails under the torrid zone a surprising stabi- 
lity of temperature. However enormous be the mass of the 
Cordilleras, it acts but feebly on the strata of air which are un- 
ceasingly renewed. On the other hand, if the plains are heated 
during the day, they radiate as much during the night ; for it 
is principally on the plains elevated 8856 feet above the sea, 
that the sky is most clear and uniformly serene. At Peru, for 
example, the magnificent plateau of Caxamarca, in which the 
wheat yields the eighteenth, and barley the sixtieth grain, has 
an extent of more than twelve square leagues : it is smooth like 
the bottom of a lake, and sheltered by a circular wall of moun- 
tains free from snow. Its mean temperature is 60°.8, yet the 
wheat is often frozen during the night ; and in a season where 
the thermometer fell before sunrise to 46°. 4, I have seen it rise 
in the day to 77° in the shade. In the vast plains of Bogota, 
which are 656 feet less elevated than that of Caxamarca, the 
mean temperature, as established by the fine observations of 
Mutis, is scarcely 57°. 74. 
In comparing towns situated on elevated plains with those 
which are placed on the declivity of mountains, I have found for 
the first an augmentation of temperature, which, on account of the 
VOL. IV. NO. 8. AriUL 1821. 
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