( 142 ) 



I = -Is 72- B(i tg \tg C I — 



— « sin C (R^ — 

 a shi C a cos C [/{R" — «") 



- j 1^0 iff 



/ a 



\/(R^ — «2) 



+ 



If C moves on the circle ABC^ tlie centre of which is M, «and 

 C remain the same and consequently I keeps its value also. 



Let provisionally, the distance of A and B remain invariable and 

 let C move arbitrarily, then M moves along the line FG. If we call 

 the heigt of M above AB equal to h, I may be considered as func- 



tion of h by observing that fr" = /r 4-—'('' the distance ^S) and 



sill C=: — . If the wholf 

 2 « 



figure is turned round line AB, and if 



