CONTINUOUS CALCULATING MACHINES. 
QQO 
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sphere when its axis of rotation is in the plane of that great circle. The only axis 
fulfilling both these conditions is that coinciding with the intersection of the two diametral 
■planes. 
Figs. 13, 14, 15, 16, 18, and 19. 
It was at the extremity of this axis C C' (fig. 17) that the movable centres were 
always in contact; but their successful action was not due to this fact, but simply 
because they, together with the supporting roller, fulfilled the other conditions. 
These conditions, which can really he approximately, but never perfectly, fulfilled, 
may be briefly stated as three in number :— 
3 d 2 
