Brard 
The other three equalities (5.16) give: 
v"(M2 ) MjM, + V"(M ) MM, = taal © a bc A) 
2 Zz 
WN ° ! = ! ° ! = 
Vv ae De ay ae M}M, Oye 
AG Euv | . MMi + [FaiMy y+ ¥ (se MIM, = 
- ta (oo ae O(7) 
The left member is equalto [ "(C.) + O(7); we have thus: 
Pies) = r(C,) + -O (ar). (5. 18) 
But IT '(C,) is equal to the flux of Z - Es through any open surface 
S, the edge of which is C,. We may take 
S = ‘rectangle M VES Vi Mi + S 
e Qe) ey 1, 1, l 
+ rectangle M' M M. M: (5. 19) 
Sa Sh tae 
where S, is any open surface the edge of which is the contour 
1216 
