War win. 
Phil. Trans. 1881 
Plate 61 
Contours of the surface, 2 z =(9-xt- 2y*)*- 20 (% + £* ' w hen r & y an both posiiwe 
2TB. The. raks of 2x imiicoutetb by the nanCbav cn. the. Qmtcnxr- Lines are ait negative, 
so that. the. SYVbodJuer numbers indicate. higher Contours. 
X 
