( 234 ) 



fig. 2. 



figure. ]t extends all over the width of the figure. Ahvays when A he^ 

 on the side of the small vohimes of an isobar, two tangents may 

 he drawn to snch a line from ]\. These tangents tonch the isobar 

 at jKjints, lying on eitiier side of the inMeetion point: and for these 

 points of eontact I'sf = 0. Another isoi)ar an ill funnsh two other 

 points of eonlact, if we leave the point /\ nnclianged. We ha\e 

 therefore for evei-y point F, a locns, eonsisting of two branches, 

 for which r,f = 0. If the point Fs lay at greater volume, i. e, on 



