JoLY — Some Froperties of the General Congruency of Cuwes. 665 



a variable quadric cone must have one edge fixed in direction. This 

 is equivalent to the system of conditions that four conies should have 

 a common point, and a system of differential equations involving the 

 differential coefficients of x, y, and % in the third order with respect 

 to u, V, and iv must be satisfied. 



YII. — Finally, some calculations are made in the somewiiat 

 manageable case of universal curves concerning the numerical 

 characteristics of congruencies of this special type. 



