368 WILSON AND MOORE. 



is an invariant of A, as verified above in a different way. Further 

 26 = i»A»j + j'A'i may by a rotation of i, j into i', j' be seen to 

 describe a conic, our indicatrix. 



These brief remarks must suffice to indicate the ease and directness 

 of the vector method of discussing surfaces through the use of the 

 dyadic A = VM which is determined by the relation c?M = dvA, 

 where V is a sort of surface differentiation built from analogy with the 

 ordinary V and defined identically with it by the equation d = dfV. 



Massachusetts Institute of Technology, 

 Boston, Mass., March, 1916. 



