RECIPROCAL SCREWS. 27 



The line joining, the points in which the ellipse is 

 cut by two intersecting screws on the cylindroid, is pa- 

 rallel to the minor axis. 



All the screws which lie in a plane and are reciprocal 

 to a cylindroid envelope a conic. The relation of this 

 conic to the cubic in which the plane cuts the cylindroid 

 might have some geometrical interest. 



As we have no physical applications to make of these 

 theorems, the demonstrations are not given. 



