LI
of the air. 263. (225). Also putrefaction as a slow process of
combustion (damp pay takes fire), 264; but for combustion
a certain amount of heat is wanted, which amount varies
with different bodies (combustible and incombustible bodies).
265.
§ 125. d) The most powerful source of heat is the radia-
tion of the sun. “Faraday calculated that the average amount
of heat radiated in a day on each acre of ground in the latitude
of London is equal to that which would be produced by the com
bustion of 60 sacks of coal.”
§ 126. e) As to terrestrial head or the heat peculiar to
our earth, the heat on the surface of the earth's crust is due sole
ly to the influence of solar radiation. In a depth of 3–5' in the
temperate zone the daily changes of temperature cease, in a
depth of 60-80' we have the layer of constant temperature and,
we may say, here ends the inflnence of the sun's radiation.
Below this layer we may observe an increase of 1° C for 129
feet. This increase is not the same everywhere; it seems to be
influenced by the condition of the various strata. Proofs of
this inward-heat are: 1. Mines (care must be taken to exclude
air whilst measuring), 2. Artesian wells. A bore near Berlin
shows the following results:
| Depth | Temp. in Réaum. | Depth. | Temp. |
|---|---|---|---|
| 700′ | 17.275°° | 1700′ | 24.74°° |
| 900′ | 18.78°° | 1900′ | 26.50°° |
| 1100′ | 21.14°° | 2100′ | 28.66°° |
| 1300′ | 21.51°° | 3300′ | 37.23°° |
| 1500′ | 23.27°° | 4042′ | 38.50°° |
Also Artesian wells prove that as to the increase of heat no
rule can be fixed. There is a place (Neuffen in Germany)
where an increase of 1°° C may be observed after a descent of 34.1'
D*
