462 



Sir William Thomson on the Stability of 



known and easily proved, is the condition of solid and fluid 

 rotating with equal angular velocity. If the stiffness of the 

 containing-vessel be small enough and its viscosity great 

 enough, it is easily seen that this final condition will be 

 closely approximated to in a very moderate number of times 

 the period of rotation in the final condition. Still we must 

 wait an infinite time before we can find a perfect approxima- 

 tion to this condition reached from our highly complex or 

 irregular initial motion. We shall now, therefore, cut the 

 affair short by simply supposing the fluid to be given rotating 

 with uniform angular velocity, like a solid within the con- 

 taining-vessel, a true figure of revolution, which we shall now 

 again consider as absolutely rigid, and consisting of cylinder 

 with perforated diaphragm and two movable pistons, as repre- 

 sented in fig. 1. 



7. Give A a sudden pull or push and leave it to itself; 

 it will move a short distance in the direction of the impulse 

 and then spring back*. Keep alternately pulling and push- 



* The subject of this statement receives an interesting experimental 

 illustration in the following passage, extracted from the Proceedings of the 



Fie:. 2. 



Royal Institution of Great Britain for March 4, 1881; being an abstract 

 of a Friday-evening discourse on " Elasticity viewed as possibly a Mode of 



