i8 77.] 
On Underground Temperature. 
181 
effect of the currents would occur towards the bottom of the 
hole, where the cold water coming down from above would 
tend to accumulate, because there would be no warm cur- 
rents coming up from below to disturb it. 
When, therefore, we enquire how far the observations 
made with the geo-thermometer can be depended upon as 
having given the true temperature of the rock in mass, two 
questions present themselves. First, could the water en- 
closed between the disks assume eventually the temperature 
of the rock mass, if the instrument were left down long 
enough ? And, secondly, if it could do so, was it in faCt 
left down long enough ? 
Before attempting to answer these questions, it may be 
well to refresh our minds regarding the mode by which a 
material like rock transmits heat through its substance, and 
by so doing affeCts the temperature of whatever may happen 
to be in contaCt with it. In such a case the heat is not 
conveyed from point to point by the fetch-and-carry system 
which, going on in a liquid like water, is called convection, 
and has been already described ; but each particle of rock 
receives heat from those in contadt with it if they happen 
to be hotter than itself, and gives up heat to those in con- 
tadt with it which are cooler than itself, without ever moving 
from its place. Thus the heat is passed on from particle to 
particle, much as we see buckets of water passed from one 
person to another at a fire. This mode of propagation of 
heat is called conduction, while the power inherent in any 
substance to transmit heat through itself is called its con- 
ductivity ; those substances which can transmit heat most 
rapidly being said to have greater conductivity than those 
which transmit it more slowly. The conductivity of ordi- 
nary rock is small ; that of rock salt is said to be compara- 
tively great. 
Could, then, the water enclosed by the geo-thermometer 
recover the temperature of the rock mass if the instrument 
were left down long enough ? It appears that this result 
could be only partially attained, however long it were left 
in the bore-hole. The convedtive currents in the water 
would affedt the temperature of the column at the upper 
and under surfaces of the enclosing disks of the instrument. 
And even if the disks were purposely made of such badly 
conducting material as to prevent any temperature effect 
from the currents being carried through the disks, still such 
effedt would be conducted round the edges of the disks, 
through the substance of the rock itself ; consequently, 
however long the instrument might have been allowed to 
remain down, the temperature of the water enclosed be° 
