788 
Proceedings of the Royal Society 
which gives the attraction of a mass of density t in terms of the 
potentials of volume distributions and surface distributions. Put- 
ting 
o' = it + jt 2 + kt 3 , 
this becomes 
Iff 
ya-ds 
-Iff 
Up . erdq 
=ff 
TJv . a~ds 
By putting cr = p, and taking the scalar, we recover a formula 
given above ; and by taking the vector we have 
Y ff VvTJpds = 0 . 
This may he easily verified from the formula 
/Pdp = V JjfXJv . yldds , 
by remembering that 
vTp = Up . 
Again if, in the fundamental integral, we put 
(T = tJJp , 
we have 
~ ff/% =ff® ■ VvVpds ■ 
5. On the Currents produced by Contact of Wires of the 
same Metal at different Temperatures. By W. Durham, 
Esq. Communicated by Professor Tait. 
At the suggestion of Professor Tait, I undertook the investiga- 
tion of the momentary thermo-electric current developed when two 
conductors or wires of the same metal are brought into contact, the 
one being at a different temperature from the other. 
Platinum was chosen as the most suitable metal to experiment 
with, in the first instance, as it is free from the interfering action 
of oxidation at high temperatures. 
