Clyst St. George.
_Curious Fact in Natural Philosophy_ (Vol. vii., p. 206.).--In reply to
ELGINENSIS I send you a quotation from Dr. Golding Bird's _Natural
Philosophy_ in explanation of this well-known phenomenon:
"One very remarkable phenomenon connected with the escape of a
current of air under considerable pressure, must not be passed
over silently. M. Clement Desormes (_Ann. de Phys. et Chim._,
xxxvi. p. 69.) has observed, that when an opening, about an inch
in diameter, is made in the side of a reservoir of compressed
air, the latter rushes out violently; and if a plate of metal or
wood, seven inches in diameter, be pressed towards the opening,
it will, after the first repulsive action of the current of air
is overcome, be apparently attracted, rapidly oscillating within
a short distance of the opening, out of which the air continues
to emit with considerable force. This curious circumstance is
explained on the supposition, that the current of air, on
escaping through the opening, expands itself into a thin disc,
to escape between the plate of wood or metal, and side of the
reservoir; and on reaching the circumference of the plate, draws
after it a current of atmospheric air from the opposite side....
The plate thus balanced between these currents remains near the
aperture, and apparently attracted by the current of air to
which it is opposed."
Dr. G. B. then describes the experiment quoted by ELGINENSIS as "a
similar phenomenon, and apparently explicable on similar principles."
(Bird's _Nat. Phil._, p. 118.)