io Proceedings. 



Ordinary Meeting, November 27th, 1894. 

 HENRY Wilde, F.R.S., President, in the Chair. 



The thanks of the members were voted to the donors of 

 the books upon the table. 



Dr. LANGDON exhibited a sample of antitoxin, the 

 alleged cure for diphtheria. 



Professor Horace Lamb, M.A., F.R.S., made a com- 

 munication on " The Stability of Steady Motion." 



This had reference to the distinction between 'ordinary' 

 and 'secular' stability, and to the criterion for the latter 

 kind of stability formulated by Thomson and Tait. A 

 simple illustration of the theory is furnished by the case of 

 a particle in an ellipsoidal bowl rotating about a principal 

 axis which is vertical. In the absence of friction there will be 

 'ordinary' stability, with the particle at the lowest point, 

 when the rotation is sufficiently slow, and again when it is 

 sufficiently rapid ; there being an intermediate stage of 

 instability, viz., when the period of the rotation lies between 

 the periods of the two principal modes of small oscillation 

 when the bowl is fixed. But if there be friction between the 

 particle and the bowl, there will be 'secular' or practical 

 instability so soon as the rotation is more rapid than the 

 slower of the two oscillations aforesaid. In this case the 

 particle, if disturbed, will work its way outwards, and 

 finally settle down into some excentric position ; the energy 

 of the system, for the same angular momentum about 

 the axis of rotation, being less than when the particle was 

 at the lowest point. Instead of a particle in a bowl we 

 might have a Blackburn's pendulum, with the upper string 

 symmetrically attached to a horizontal bar which is made 



