251 



3. x'y and xy^ present, other terms present or absent, 



4. x^y and y- present, other terms present or absent (or xy- and x^), 

 6. X* andy2 present, other terms of lower degree present or absent. 



1 and 2 are built from 4tli degree terms and tlie resulting equation 

 is only the 4th. 



3, has two 3d degree terms present, scroll 4tli. 



4, one term 3d degree, other 2d, scroll 4th. 



5, built from second degree terms, scroll 4th. 



Fig. 3. 



Fig. 3, shows at once the order of the resulting scroll when the equa- 

 tion of the curvilinear directrix is marked by the presence or absence of 

 certain specified terms. 



