TEMPERATURE OP THE INTERIOR. 85 
Corclier in the coal mines of the south, the middle, and the north 
of France, by De Trebra in Saxony, Humboldt in Mexico, and 
Daubuisson in Saxony and Britanny. Coal mines have one ad- 
vantage over every other for this kind of investigation. The 
excavations being carried rapidly forward, there is little oppor- 
tunity for the foreign sources of heat just mentioned, to operate, 
so that the temperature at the point where the coal is being 
taken, represents trul} r , that of the part of the earth's crust in 
which it lies. 
In the deepest coal mine in Great Britain, at a point 15S4 feet 
below the level of the ground, and 1500 beneath the level of the 
sea, the thermometer on the 15th of Nov. 1S34, stood at 65° in 
the air close to the coal, and at 71°2. when left in a hole bored in- 
to the coal, for a week, the temperature of the day of observation 
being 49° and the mean temperature of the surface, 47°6. 
The rate of increase is different at different places, depending 
as is supposed upon the greater or less conducting power of the 
strata and other unknown causes. One degree of Fahrenheit for 
45 feet of descent may be assumed as an approximation to an 
average. At this rate, the temperature of boiling water (212°) 
will be found 6795 feet, or somewhat less than a mile and one- 
third beneath the surface at Chapel Hill, and a heat intense 
enough to fuse the rocks at a depth of between 50 and 60 miles. 
We have no data for forming even a probable conjecture respect- 
ing the temperature of the centre. If the interior is an homoge- 
neous fluid, it is obvious that uniformity of temperature would be 
produced by the currents that would be established in it. If it is 
composed of metals, ranged in the order of their specific gravi- 
ties, no certain inferences can be drawn from observations made 
in the exterior crust. 
2. When the surface of the earth is occupied by tertiary de- 
posits, a supply of good water, is often not to be had at moderate 
depths, and it becomes necessary to bore through the upper strata, 
which either yield no water at all, or such as is unfit for use, by 
reason of the quantity of salts of different kinds that it holds dis- 
solved. These deposits are commonly arranged in successive 
layers around the sides and over the bottom of a basin, which is 
frequently of no great size or extent. When a stratum of pure 
sand intervenes between two strata of clay, the former yielding 
almost a free passage, whilst the latter are nearly impervious to 
water ; if a hole 3 or 4 inches in diameter be driven by boring, 
from the surface, through the uppermost bed of clay, to some dis- 
tance into the sand; and a casing of tin, copper, or lead introduced 
to prevent the ingress of water from the strata that are penetrated, 
an abundant supply of excellent water is often obtained. This is 
raised by hydrostatic pressure, exerted around the sides of the 
basin, to a greater or less height along the tube, sometimes to the 
surface, and in many cases it is seen to overflow at the surface, 
forming a copious and perennial spring. Having been first known 
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