816 
ME. W. HOPKES’S’S EXPEBEMEXTAX EESKERCHES 
general case, placed indefinitely near to the same terminal plane. But we shall then 
have, in the limits 
^2 
&c. 
fjt) f{n) . 
and consequently the alternate equations of the system of equations (/3.)(art. 8) will become 
t\ — t" r), 
&c. = &;c. 
Adding, we have 
fi — t). 
The conductivity of the actual mass being k, and the thickness H, we have 
k ■ 
(») 
,(n) 
• t2 
4”’-t 
(art. 1) ; 
and k' being the conductivity of a mass of the same thickness, and of which the terminal 
temperatures are, by hypothesis, 4 and we have 
pYL U-tf 
k< ~ ff-r' 
Hence 
and 
k' 1 
k p k ’ 
q pYi 
as before. 
10. Effect of Moisture. 
The following experiments were made to ascertain the influence of moisture on the 
conductive powers of rocks. 
(1) Calcareous Bocks. — I took a block of chalk from the lower part of the Chalk 
formation near Cambridge. It is provincially called cluncli, and is frequently used as 
building-stone, where it can be well defended against the disintegrating influences of the 
