14 Temperature of the Terrestrial Globe. 
We must ascertain, for example, whether the temperature increases 
with the depth as we go below those points whose diurnal and an- 
nual variations have ceased to be sensible. Now all observations 
collected and examined by the most learned philosophers of our day, 
show us that such an increase actually exists. It has been estimated 
at about one degree for thirty or forty metres. The experiments 
with which we have lately entertained the Academy, relating to the 
heat of springs, confirm the results of preceding observations. 
The object of the inquiry we propose, is to discover the certain 
consequences of this single fact, admitting it to be given by direct 
observation ; and to prove that it determines, first, the situation of 
the source of heat, and, secondly, the excess of temperature which 
still exists at the surface. 
It is easy to conclude, (and the same result is obtained from an 
exact analysis,) that the increase of temperature as we go towards 
the centre cannot be produced by a prolonged action of the sun’s 
rays. ‘The heat proceeding from that body is accumulated in the 
interior of the earth ; but the progress has almost entirely ceased, 
and if the accumulation was still continued, we should observe that 
increase in a direction precisely contrary to what we have mentioned. 
The cause which gives to deep strata a higher temperature, is, 
therefore, an internal source of constant or variable heat, situated be- 
low where man has been able to penetrate. This cause raises the 
temperature of the surface above what it would have been from the 
simple action of the sun’s rays. But this excess of temperature of 
the surface has become almost insensible. Of this we are assured, 
because there exists a mathematical ratio between the value of in- 
crease by metre and the quantity, by which the temperature of the 
surface still exceeds that which would be found, if the internal cause 
of which we are speaking did not exist. 'To measure the increase 
by unity of depth, is the same thing as to measure the excess of 
temperature at the surface. 
In a globe of iron the increase of a thirtieth of a degree per me- 
tre, would only give a fourth degree of actual elevation of tempera- 
ture at the surface. This elevation is in direct ratio to the condact- 
ing power of the substance of which the crust is formed, all other 
conditions remaining the same. Thus the excess of temperature, 
which the terrestrial surface has at present, in consequence of this 
internal source, is very small; it is probably below the thirtieth of a 
centesimal degree, (,';° Fahrenheit.) We ought to remark that 
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