370 
Proceedings of the Royal Society 
Monday , *Ith December 1885. 
JOHN MURRAY, Esq., Ph.D., Vice-President, 
in the Chair. 
Two instantaneous Photographs — one of the explosion of Hellgate, 
and the other of the “ Genista” yacht — were exhibited by the 
Astronomer-Royal for Scotland. The thanks of the Society were 
accorded to the Astronomer- Royal. 
The following Communications were read 
1. On the Motion of a Liquid within an Ellipsoidal Hollow. 
By Sir William Thomson. 
I have only recently noticed the propositions regarding fluid 
motion within an ellipsoidal hollow which form the subject of the 
present communication, and which, though obvious enough and 
remarkably interesting, do not seem to have been previously dis- 
covered. 
Preliminary. 
I shall use the expression homogeneous rotation, or homogeneous 
molecular rotation, to designate the condition of a fluid in respect to 
rotation, when throughout it the amounts of its molecular rotation 
are the same and the axial lines parallel. This designation clearly 
includes the case of a rotating solid : but it is applicable of course 
to the more complex case of a fluid, in which irrotational motion is 
superimposed upon homogeneous rotation as of a solid. To illustrate 
the complex motion thus signified, consider the following three 
examples, of which (1) and (2) are included in (3): — 
(1) Let a liquid kept in the shape of a figure of revolution, 
by a rigid containing vessel, be given in a state of homogeneous 
rotation round the axis of the figure. Let an impulsive rotation 
round a line perpendicular to this axis be given to the containing 
vessel. The instantaneous motion of the liquid, at the instant when 
the impulse is completed, consists of an irrotational motion super- 
imposed on the given homogeneous rotational motion. The 
molecular rotation of the liquid does not generally remain homo- 
