porous vessel at whose base is fixed a glass tube that dips into
water. The hydrogen, in diffusing, enters the porous vessel, increases
the internal pressure, and a number of bubbles escapes from the tube.
On withdrawing the bell glass of hydrogen, the latter becomes diffused
externally, a lower pressure occurs in the porous vessel, and the
level of the water rises.

The arrangement devised by Mr. C.J. Woodward, and recently presented
to the Physical Society of London, is an adaptation of this experiment
to the production of an oscillating motion by alternations in the
internal and external diffusion of the hydrogen.

The apparatus, represented herewith, consists of a scale beam about
three feet in length that supports at one end a scale pan and weights,
and, at the other, a corked porous vessel that carries a glass tube,
c, which dips into a vessel containing either water or methylic
alcohol. Three or four gas jets, one of which is shown at E, are
arranged around the porous vessel, as close as possible, but in such a
way as not to touch it during the oscillation of the beam. These gas
jets communicate with a gasometer tilled with hydrogen, the bell of
which is so charged as to furnish a jet of sufficient strength.
Experience will indicate the best place to give the gas jets, but, in
general, it is well to locate them at near the center of the porous
vessel when the beam is horizontal.

[Illustration]

It is now easy to see how the device operates. When the hydrogen comes
in presence of the porous vessel it becomes diffused therein, and the
pressure exerted in the interior then produces an ascent. When the