of Edinburgh, Session 1869-70. G3 
irrespectively of tlie direction of the stream lines in its neighbour¬ 
hood ; a result easily deduced from the elementary formula for fluid 
pressure in hydrokinetics. 
I have long ago shown that an elongated diamagnetic body in a 
uniform magnetic field tends, as tends an elongated ferromagnetic 
body, to place its length along the lines of force. Hence a long 
solid, pivoted on a fixed axis through its middle in a uniform stream 
of liquid, tends to place its length perpendicularly across the direc¬ 
tion of motion ; a known result (Thomson & Tait’s “ Natural Philo¬ 
sophy,” § 335). Again, two globes held in a uniform stream with 
the lines joining their centres, require force to prevent them from 
mutually approaching one another. In the magnetic analogue, two 
spheres of diamagnetic or ferromagnetic inductive capacity repel 
one another when held in a line at right angles to the lines of 
force. A hydrokinetic result similar to this for the case of two 
equal globes, is to be found in Thomson and Tait’s “ Natural Philo¬ 
sophy,” § 332. 
IV. (Proposition.) If the second body considered in § III., that is 
to say, a body either having no apertures, or, if perforated, having 
no circulation through the apertures, be acted on by one system of 
forces applied so as always to balance the resultant of the fluid 
pressure, calculated for it according to II. and III. for whatever 
position it may come to at any time, and if it be influenced, besides, 
by any other system of applied forces, superimposed on the former, 
it will move just as it would move, under the influence of the latter 
system of forces alone, were the fluid at rest, except in so far as 
compelled to move by the body’s own motion through it. A parti¬ 
cular case of this proposition was first published many years ago, by 
Professor James Thomson, on account of which he gave the name 
of “ vortex of free mobility ” to the cyclic irrotational motion sym¬ 
metrical round a straight axis. 
4. On the Equilibrium of Vapour at a Curved Surface of 
Liquid. By Sir William Thomson. 
In a closed vessel containing only a liquid and its vapour, ail at 
one temperature, the liquid rests, with its free surface raised or 
depressed in capillary tubes and in the neighbourhood of the solid 
boundary, in permanent equilibrium according to the same law of 
