SURFACES IN HYPERSPACE. 273 



We shall not make a direct use of that theorem but shall give an 

 independent demonstration of the special case in which we are inter- 

 ested. For in order properly to understand the statement of this 

 theorem it would be necessary to explain the technical language of 

 Ricci's absolute differential calculus, and we consider it better to 

 explain this piece by piece as we need it, and to give at length demon- 

 strations of theorems which are special cases of his, in order that we 

 may make the work less abstrf»ct. 



