253 



It will frequently hapiien that the equation of a scroll will be sought 

 whose three directrices are given as above, viz., two rectilinear and one 

 plane curvilinear directrix, but the latter in some plane not parallel to 

 the two former lines. 



In this case additional means should be given for writing the equa- 

 tion of the surface undex the new conditions. It will then be easy to 

 find a section parallel to the two right-line directrices and the problem 

 then is solved by the process discussed in this paper. 



A modification of the method here discussed finds the equation of a 

 scroll given by two rectilinear directrices and a plane section of the 

 surface, the section being oblique to a plane parallel to the two given 

 straight lines 



