258 PROCEEDINGS OF SECTION A. 



4.— ON THE BULGING OF FLAT PLATES. 



By J. H. MICHELL. 



[^Abstract.^ 



The author investigated the general differential equations by 



means of which the instability of a plate in the flat form 



under boundary stress in its plane is determined. 



If P, Q, U in the elements of stress at the point ix, y) in 

 the flat form, and a the normal displacement, the areal 

 differential equation is — 



2A A 4,, - ±-(v^ X U^#') - f (q^ + U^l = 

 ax \ ax ay J ay \ ay ax I 



d^ d^ 



where A» = ^, + g-.. 



In particular the symmetrical bulging of a circular plate 

 under uniform radial pressure was worked out. The result 

 arrived at was the following comparison : — 



Suppose the circular disc to be subjected to uniform 

 radial pressure P per unit length, and a strip of the 

 same material and of unit breadth has forces P applied 

 at the ends : then the critical radius of the disc is 

 nearly two-thirds the critical length of the rod, — 

 supposing Poisson's ratio to be one-fourth. 



5.— THE TASMANIAN EARTH TREMORS. 



Bxj A. B. BIGGS. 

 \^Abstract.'\ 

 This paper discusses the share which our island has had 

 in the general seismic disturbances throughout the world 

 which has characterised the decade just passed, taking up 

 our tremors from the initiation of the series about May, 

 1883, to its apparent cessation within the last year. 



The most noteworthy shocks were given, in the order of 

 dates, as follows:— 13 December, 1883; 27 May, 13 July, 

 19 September, 1884; and 13 May, 1885,— this last being 

 considered the chmax of these series. 



The two questions which would naturally press for solution 

 at the earliest stage of this new experience would be : — 1st, 

 as to the actual magnitude of the shock ; and 2nd, its 



