62 THE PRINCIPAL SCREWS OF INERTIA. 



screws of reference. The six co-ordinates of a, with re 

 spect to the same screws, are therefore 



&C. 



We can now express the condition that a and ]3 are con 

 jugate screws of inertia. This condition is ( 54) 



o. 



Denoting p?A? + . . . + p & A & * by uS, and observing that 

 2/iVl i^ t and similar expressions are zero, we deduce 



