18 Dr. D. F. Comstock on the 
refers to a point fixed in space, and if we wish it to refer 
to a point moving with the system we must write as usual 
dT=^-^-' < 24 > 
where -^— now means the rate of change measured from the 
ot 
moving point. Likewise, if we wish the velocities which 
enter into (23) to be expressed in terms of velocities relative 
to axes moving with the system, we must write 
%=*>2y r (25) 
V z = l'2z J 
where v 2x , v 2]/ , and v 2z are the components of these relative 
velocities. 
Substituting (24) and (25) in (23), and remembering the 
simple proportionality between S z , S y , and S z and the 
density of momentum m x , my, and m z , we easily obtain 
-(v 2x p$ x + v 2y p$ } y +v 2z p!?' z ). . . (26) 
Now (p$x) may be expressed in terms of the electric and 
magnetic force intensities, together with the density of the 
momentum. This involves only the fundamental equations 
of electromagnetic theory and has been done in paragraph 7, 
reference to which will show that with the present notation 
9V +vj^ (27) 
d« ' da- 
Substituting this for the (pftx) which occurs on the left- 
hand side of (26), rearranging the latter, and putting 
M, = .-L { X 2 + Y 2 + Z 2 + * 2 + /3 2 + 7 2 } 
and »„ ( =i {y 2 + Z2 + /3 2 + 7 2 ;, 
8 
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