CORRIGENDUM. 
VOL. XXXVI. 
In Vol. XXXVI of the Proc. R.S.E., on p. 238, delete lines 17 and 18 
from the foot, and read — 
Thus we have 
and line 1 1 from the foot — 
for 
2 dt 2 
read t 
dfP 
dt 2 ' 
