i&2 On Underground Temperature. [April, 
tween its disks would have been to some extent affedted by 
the convedtive currents ; and their tendency in the lower 
parts of the column would be to reduce the temperature, 
and diminish the rate of increase. 
A method of obviating this source of error is mentioned 
in the British Association Report, already referred to, as 
having been lately suggested by Sir W. Thomson. It con- 
sists in using a series of india-rubber disks placed at consi- 
derable distances apart. 
Putting the above-discussed source of error aside, we 
come to our second enquiry, whether the geo-thermometer 
was left down long enough for the water between its disks 
to assume the temperature of the rock mass ?* Let us, then, 
consider the conditions of the system before the geo-thermo- 
meter is introduced. We have a very long vertical column 
of water enclosed in a cylindrical hole within a mass of 
rock, the rock extending to an infinite distance both side- 
ways and downwards. The water in the bore-hole at any 
given depth has, in consequence of the currents, a temper- 
ature which differs from that of the rock on the same 
horizon at a distance from the hole. This temperature of 
the water may be higher or lower than that of the rock ; 
but we will suppose it lower, as it will be in the deeper 
parts of the hole. The rock surface of the bore-hole, being 
constantly laved by the water, has been brought to the 
same temperature as the water. It necessarily follows from 
this that if we could examine the temperatures of the rock 
at greater and greater distances from the bore-hole, we 
should find them become higher and higher, and we 
should have to penetrate to some considerable distance 
before the increase came to an end, and when it did so we 
should feel assured that at last we had reached rock of the 
true temperature for the depth. The greater the difference 
between the temperatures of the water and the rock in 
mass, the further we should have to penetrate for that pur- 
pose, and this would be furthest in the lower portions. Now 
suppose the currents interrupted by lowering the geo- 
thermometer, so that the heat ceases to be conveyed away 
from the rock-surface of the bore-hole; and we will now 
dismiss the consideration of the effedt of the water above 
and below the disks of the instrument. The heat which 
* It appears that at the depth of noo feet an observation of ten hours 
duration gave the same result as one of nineteen hours ; whence it was con- 
cluded that ten was long enough. But considering the great thermal capacity 
of water, and the small changes which, except just at first, might be expedted 
in the rock, as well as the difficulty of the operation, this trial can hardly be 
held conclusive against the probability of a further increase of temperature, if 
the geo-thermometer had remained down longer. 
