8 Dr. D. F. Comstock on the 
Since 
[E, curlE],= -[E, |^, H]^, 
the last two terms in the bracket of (9) become together 
which is minus the time rate o£ the density of momentum at 
the point. The time rate, however, refers to a point fixed in 
space, and to change to a point moving with the system we 
make use of the usual expression and write 
B <y . fj'bmx . ~dm x , "dm A , . 
T^ = "5 %+ TF +m ¥ +/ ^)' (I0) 
where m x is the ^'-component of the density of momentum 
and (I, m, n) are, as formerly, the direction cosines of the 
constant velocity (r a ). The operator ^— now refers to the 
rate of change at a point moving with the system. 
Substituting (10) for the two last terms in (9) and noticing 
that 
"d'mx , . ~d d' C 7 
-~— may be written r-^- )»¥•'', 
Ot d# Ot Ir 
where the integration is to be taken from R (meaning merely 
from a point outside the system where m x is zero) up to the 
point P in question, account being taken of any discontinuity 
at the bounding surface, we have in place of (9) 
dX x dX„ dA. at 
s{-icw-v-iw-K(H--H;-ttr)-iK+|-;j.*-^} 
+ l,{-^ (E ^ +H ' H ->-^^ + lf B pm ^} (11) 
Thi s equation gives us what we were seeking, namely, the 
values of X x , X y , and X. in terms of the electric and magnetic 
forces and the density of the momentum. 
