194 Sir William Thomson. [Nov. 26, 



November 26, 1891. 

 Mr. JOHN" EVANS, D.C.L., LL.D., Treasurer, in the Chair. 



A List of the Presents was laid on the table, and thanks ordered for 

 them. 



Pursuant to notice, Alexander Agassiz, Dr. Benjamin Apthorpe 

 Gould, Professor Eduard Strasburger, and Professor Pietro 

 Tacchini were balloted for and elected Foreign Members of the 

 Society. 



In pursuance of the Statutes, notice of the ensuing Anniversary 

 Meeting was given from the Chair, and the list of Officers and Council 

 nominated for election was read as follows : 



President. Sir William Thomson, D.C.L., LL.D. 

 Treasurer. John Evans, D.C.L., LL.D. 



f Professor Michael Foster, M.A., M.D. 

 Secretary.- j ^ Lord Rayleigllj M A ^ D c L 



Foreign Secretary. Sir Archibald Geikie, LL.D. 



Other Members of the Council. Captain William de Wiveleslie 

 Abney, C.B. ; William Thomas Blanford, F.G.S. ; Professor Alexander 

 Crum Brown, D.Sc. ; Professor George Carey Foster, B.A.; James 

 Whitbread Lee Glaisher, D.Sc. ; Frederick Ducane Godman, 

 F.L.S. ; John Hopkinson, D.Sc. ; Professor George Downing Liveing, 

 M.A.; Professor Joseph Norman Lockyer, F.R.A.S. ; Professor 

 Arthur Milnes Marshall, D.Sc. ; Philip Henry Pye-Smith, M.D. ; 

 William Chandler Eoberts- Austen, F.C.S. ; Professor Edward Albert 

 Schafer, M.R.C.S. ; Sir George Gabriel Stokes, Bart., M.A. ; Pro- 

 fessor Sydney Howard Vines, M.A. ; General Jam es Thomas Walker, 

 C.B. 



The following Papers were read : 



I. "On Instability of Periodic Motion." By Sir WILLIAM 

 THOMSON, P.R.S. Received November 12, 1891. 



1. Let Y--J 0, %, $ be generalised coordinates of a system ; and let 



A (-^, 0, , ->^', $', ) be the action in a path ( 2 above) from 



the configuration (y/, 0', . . . . ) to the configuration (^, 0, . . . . ) with 

 kinetic energy (E V) with any given constant value for E, 

 the total energy; V being the potential energy, of which the 



