r the whole earth, being twice or three
times as much in one country as in another, and these differences are
not in constant relation either with the latitudes or longitudes of
places.[750] He is of opinion, however, that the increase would not be
overstated at 1 degrees Cent. for every twenty-five metres, or about 1
degrees F. for every forty-five feet.[751] The experimental well bored
at Grenelle, near Paris, gave about 1 degrees F. for every sixty English
feet, when they had reached a depth of 1312 feet.

Some writers have endeavored to refer these phenomena (which, however
discordant as to the ratio of increasing heat, appear all to point one
way) to the condensation of air constantly descending from the surface
into the mines. For the air under pressure would give out latent heat,
on the same principle as it becomes colder when rarefied in the higher
regions of the atmosphere. But, besides that the quantity of heat is
greater than could be supposed to flow from this source, the argument
has been answered in a satisfactory manner by Mr. Fox, who has shown,
that in the mines of Cornwall the ascending have generally a higher
temperature than the descending aerial currents. The difference between
them was found to vary from 9 degrees to 17 degrees F.; a proof that,
instead of imparting heat, these currents actually carry off a large
quantity from the mines.[752]

If we adopt M. Cordier's estimate of 1 degrees F. for every 45 feet of
depth as the mean result, and assume, with the advocates of central
fluidity, that the increasing temperature is continued downwards, we
should reach the ordinary boiling point of water at about two miles
below the surface, and at the depth of about twenty-four miles should
arrive at the melting point of iron, a heat sufficient to fuse almost
every known substance. The temperature of melted iron was estimated at
21,000 degrees F., by Wedgwood; but his pyrometer gives, as is now
demonstrated, very erroneous results. Professor Daniell ascertained that
the point of fusion is 2786 degrees F.[753]

[Illustration: Fig. 92.

Section of the earth, in which the breadth of the outer boundary line
represents a thickness of 25 miles; the space between the circles,
including the breadth of the lines, 200 miles.]

According to Mr. Daniell's scale, we ought to encounter the internal
melted matter before penetrating through a thickness represented by
that of the outer circular line in the annexed dia