On Underground Temperature. 
[April, 
than ten hours, the temperature shown by it was 
assumed to be that of the rock at the corresponding depth. 
This was found to differ sometimes by more than a degree 
Reaumur from the temperature of the water at the same 
depth, as shown by the first-named thermometer, when the 
water above and below was not cut off. Now, it can hardly 
be supposed that so great a difference as this really existed 
simultaneously between the surface of the rock exposed to 
the water in the bore-hole and the water itself. On the 
contrary, it seems evident that the surface of the rock, 
being exposed to the action of the water, must have assumed 
the temperature of the water, whatever that might be, at 
any given depth. But the water was undoubtedly affected 
by convention currents, and consequently its temperature 
was not the same as that of the rock in mass at a distance 
from the bore-hole. Hence, when the circulation of these 
currents was stopped by the disks, the surface of the bore-hole 
would begin to tend towards the true rock temperature. If 
the water at any depth was warmer than the rock in mass, 
its temperature would begin to fall when the currents were 
cut off, and if the water was cooler than the rock it would 
begin to rise. 
Let us, then, shortly consider the general effedt of com 
vedfive currents as they would affedl the water. 
These currents arise from the circumstance that when 
water is warm it expands, and consequently becomes lighter. 
This expansion is exceedingly small ; yet in a substance of 
such extreme mobility as water it is sufficient to cause the 
expanded portions to rise, while the denser portions above 
sink to supply their place. The expanded portions in rising 
carry their heat up with them, and the cooler portions in 
sinking reduce the temperature of the lower part of the 
column. If therefore we had a column of water originally 
warmed towards its lower portion, but not supplied there with 
fresh accessions of heat, the whole would, if open to the atmo- 
sphere, shortly assume an equable temperature throughout. 
Let us now invoke the aid of a diagram to render our 
ideas more clear ; and suppose the depths in the bore-hole to 
be measured along the vertical line O X, and let lines drawn 
at right angles to this represent, in proportion to their 
lengths, the temperatures at the corresponding depths. 
Let O A represent the mean temperature of the surface of 
the ground. Then on the supposition that the temperature 
of the rock in mass increases proportionally to the increase 
of depth, the temperature of the rock will be represented by 
a straight line, as A C. If, then, the temperature of the 
