1877-] Un Underground Temperature. 185 
The argument, we are told, has been used by a geologist that, 
because the increase has been found proportional to the in- 
crease of depth as far as observations have gone, “ it is most 
in accordance with inductive science to admit no great devia- 
tion in any part of the earth’s solid crust ” from the law of 
proportionality that has been observed as far down as ob- 
servations have extended. The truth seems to be that the 
uniformitarian principle had led this geologist (whose name 
is not mentioned) to argue from the known to the unknown 
according to an unvarying law ; and also that he held the 
opinion that the interior of the earth was fluid. According 
to this assumption his conclusion might not be far from 
correct, because the condition requisite for a uniform rate of 
increase is simply that a fixed temperature should be main- 
tained at a fixed depth. Now, if the interior of the earth 
were fluid, and affedted by convedtive currents, such would 
be very approximately true. For a fall of temperature 
could not take place at the fixed depth without equally 
taking place throughout the fluid nucleus. And it is obvious 
that an enormous lapse of time would be necessary to 
reduce the temperature of so vast a reservoir, by any sensible 
amount, by means of the small secular loss which takes 
place through the outer crust. Although, then, the sarcasm 
is not without point, that geologists as a race employ no 
higher method in their calculations than the “ Rule of 
Three,” nevertheless, upon the hypothesis probably in this 
geologist’s mind, that was indeed the proper rule to use, 
and the conclusion would have been corredt that the central 
fluid would be found at some depth within the limits named. 
Sir W. Thomson’s mode of reckoning the rate of increase 
proceeds upon a different plan. From astronomical and 
tidal considerations he has satisfied himself that the earth 
has not a fluid nucleus, and, from his mode of viewing the 
manner of its consolidation, he believes that it passed from the 
state of a fluid to that of a solid globe in a comparatively 
short space of time ; since which period all geological 
events have happened. He has adapted a mathematical 
formula to express the rate of increase of temperature, at 
any given depth, in terms of the length of time since this 
complete consolidation took place ; and by means of this 
formula he performs those seemingly marvellous computa- 
tions which are so astonishing to the uninitiated. Now, it 
is a remarkable fadt that, for reasonably small values of the 
time in question, — in other words, of the age of the habit- 
able world, — and for all such depths as the puny efforts of 
man have been able to reach, it would be impossible to 
perceive any deviation from a uniform rate of increase. But 
