Ball — Dynamical Problems in the Theory of a Rigid Body. 35 



ADDENDUM [5^ June, 1883.] 



[Read, 25th June, 1883.] 



I HAVE for some time been seeking to obtain a clear geometrical con- 

 ception of tbe intensity of the impulsive wrench which is capable of 

 giving the unit of twist velocity on the instantaneous screw. I desired 

 to connect this with the geometrical illustration which forms the basis 

 of the present Paper. It was only within the last few days that I 

 noticed the solution of the problem, which it is the object of this note 

 to communicate. 



Fis. 3. 



Let P, Q, R, S be points on the circle corresponding to four im- 

 pulsive screws, and let P', Q', R', S' be the four corresponding instan- 

 taneous screws deduced by the construction already given. Let f^, q, 

 r, s denote the intensities of the impulsive wrenches on P, Q, R, 8, 

 which will give the units of twist velocity on P', Q', R, S'. Suppos- 

 ing that impulsive wrenches on P, Q, R neutralize, then the corre- 

 sponding twist velocities generated on P', Q', R must neutralize also. 

 In the former case, the intensities must be proportional to the sides of 



