66 

 But 



and 



Journal of the Mitchell Society 



[Di'xember 



) 



Hence 



dx^ aV a^y^ ' 



which is negative certainlj^ for the values 

 a b 



Consideration of the other combinations of signs will be deferred 

 until the last method is reached. 



Method IV 



We may deal wath the problem by expressing U as an explicit 

 function in either x or y. Thus, solving (2) for y, we have 



y = + — Va- — X-, 

 a 



