30 
M. Humboldt on Isothermal Lines, 
At Enontckics, in 68J° of Lat. the difference between the 
mean temperatures of the earth and the air, is so great as 7°.74* 
Analogous differences are observed on the back of the Alps, at 
the altitude of 1400 metres (4593 feet). 
In the following small table, I have added the mean tempe- 
rature of the atmosphere, by supposing, with M. Ramond, 
that there is a decrease of 1° centigrade for 164 metres (1° Fahr. 
for 600 feet nearly), and by placing the temperature of 63° (ac- 
cording to observations made at the Hospice of St Gothard), at 
1950 metres (6396 feet) of elevation. 
Alt. in 
Temperature. 
Feet. 
Springs. 
Air. 
Rigi Kaltebad, 
4717 
43°. 7 
38°. 12 
Pilate, 
,4858 
41.0 
37,40 
Blanche Alp, 
5786 
37.4 
35.78 
Rossbaden, 
7016 
38.3 
31.38 
It may be objected, that in 
the Alps 
of Switzerland, the tern- 
perature of springs has only been observed from 
the beginning 
of J une to the end of September, and that the differences be- 
tween the air and the interior of the earth would almost entire- 
ly disappear, if we knew the temperature of the springs during 
the whole year. It must not be forgotten, however, that the 
springs of the Alps did not vary in the space of four months at 
the time of the observations of M. Wahlenberg ; — that among 
the small number of scanty springs which indicate changes of 
temperature in different seasons, these variations amount from 
June to September to 11° or 15°; — and that several springs, 
particularly those which are very copious, do not vary during 
a whole year more than half a degree of Fahrenheit. 
It appears to me, therefore, sufficiently certain, that where 
the earth is covered with a thick stratum of snow, while the 
temperature of the air descends to 15° or — 4° of Fahrenheit, 
the temperature of the earth is above the mean temperature of 
the air. 
When we consider what a large portion of the globe is cover- 
ed with the sea, and examine the temperature of the deepest 
waters *, we are constrained to admit, that in islands, along 
* At Funchal in Madeira, the temperature of caverns appears to be and 
consequently 7°. 2 below that of the air — - Phil . Trans. 1778, p. 37?. 
I 
