14 



Now, if we differentiate the expressions, 



w 



J't) 



J« ~ "^ s»^ 



+ y 





+ " 



SfV _ 



6xdy 





rv 



dj^oy 



+ 2 



J'V 



(K) 



which result from the foregoing number, and put for abridgment, 





**-J ^-r + J/3.J -.TTT- + h-i -SIT-; = * ST' 



5»« 





a*Sy 



^-* i^ + ^'-^ w + *^-^ m 



— j» 



ii"' 







1^ 



3u 





cv 



??• 



we find 



'■'^-(''•&+«»-&+*i^) = "&+w|+w'|. 



= m^+VSj^ + m|. 



and therefore, by (I), 



