20 M. Humboldt on Isothermal Lines, and Distribution of Heat. 
ing curve. In the morning the true mean temperature, such as 
we find by taking the duration into account, is a little greater 
than half the sum of the extremes In the evening the error 
is in a contrary direction, and the series of temperatures ap- 
proaches more to a progression by quotients. The differences 
do not in general exceed half a degree, and calculation proves 
that the compensation is regular. It would be curious to exa- 
mine the effect which the radiation of the earth has on these ho- 
rary effects, as the changes of temperature at the surface do 
not follow the geometrical progression, in so far as they take 
place in a medium of uniform temperature. 
In order to avoid the use of an arbitrary measure, astrono- 
mers express the diameters of the planets by taking that of the 
earth for unity. In like manner, I express the mean tempera- 
tures not in parts of the equatorial heat, but by the arithmetical 
ratios which subsist between this heat and that of the other paral- 
lels. This method frees us from the want of uniformity, which 
arises from the use of different thermometers. Instead of say- 
ing that in Europe, under the parallel of 45°, the mean tempe- 
rature is 13°. 4 Centigrade, or 56°. 12 of Fahrenheit, we say that 
it is ==: 1.0°, 487, and in lat. 55° 1.0°,29. These arithmetical 
ratios inform us of what is most interesting in the theory of the 
distribution of heat, that in thermometers whose zero is the 
point of melting ice, the mean temperatures under the latitude 
of 45° and 55° are, in our regions, the half and the third near- 
ly of the equatorial temperature, which I suppose to be 81°.5. 
( To he continued.) 
* Example . — Latitude 10°.25^ 
Calculation of a 
Supposition of 
true mean by 
an arithmetical 
the duration. 
progression. 
Before the maximum, 11th September 1799, 
70°. 52 Fahr. 
69°.44 Fahr. 
14 th 
69.26 
68,00 
18th 
71.24 
70.34 
After the maximum, 
18th August, 
68.72 
69.80 
20th 
70,16 
71.24 
27th 
68.72 
69.26 
Before the maximum. 
17th August, 
69.26 ' 
68.00 
After the maximum. 
17th August 
^5.48 
66.02 
Total effect, 
17th August, 
67.37 
67.01 
H. 
