PROCEEDINGS 
OF THE 
ROYAL SOCIETY OF EDINBURGH. 
vol. ix. 1875-76. No. 91. 
Ninety-Third Session. 
Monday , 2CBA December 1875. 
Sir WILLIAM THOMSON, President, in the Chair. 
The following Communications were read : — 
1. Yortex Statics. By Sir William Thomson. 
(Abstract.) 
The subject of this paper is steady motion of vortices. 
1. Extended definition of “steady motion.” The motion of any 
system of solid or fluid or solid and fluid matter is said to be steady 
when its configuration remains equal and similar, and the velocities 
of homologous particles equal, however the configuration may 
move in space, and however distant individual material particles 
may at one time be from the points homologous to their positions 
at another time. 
2. Examples of steady and not steady motion 
(1.) A rigid body symmetrical round an axis, set to rotate round 
any axis through its centre of gravity, and left free, performs 
steady motion. Not so a body having three unequal principal 
moments of inertia. 
(2.) A rigid body of any shape, in an infinite homogeneous liquid, 
rotating uniformly round any, always the same, fixed line, and 
moving uniformly parallel to this line, is a case of steady motion. 
(3.) A perforated rigid body in an infinite liquid moving in the 
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