gested by this appliance.
It enables two lights to be compared by the relative brightness of their
reflections in a silvered bead, which describes a narrow ellipse, so as
to draw the spots into parallel lines.

In 1828, Wheatstone improved the German wind instrument, called the MUND
HARMONICA, till it became the popular concertina, patented on June 19,
1829 The portable harmonium is another of his inventions, which gained
a prize medal at the Great Exhibition of 1851. He also improved the
speaking machine of De Kempelen, and endorsed the opinion of Sir David
Brewster, that before the end of this century a singing and talking
apparatus would be among the conquests of science.

In 1834, Wheatstone, who had won a name for himself, was appointed to
the Chair of Experimental Physics in King's College, London, But his
first course of lectures on Sound were a complete failure, owing to an
invincible repugnance to public speaking, and a distrust of his powers
in that direction. In the rostrum he was tongue-tied and incapable,
sometimes turning his back on the audience and mumbling to the diagrams
on the wall. In the laboratory he felt himself at home, and ever after
confined his duties mostly to demonstration.

He achieved renown by a great experiment--the measurement of the
velocity of electricity in a wire. His method was beautiful and
ingenious. He cut the wire at the middle, to form a gap which a spark
might leap across, and connected its ends to the poles of a Leyden jar
filled with electricity. Three sparks were thus produced, one at either
end of the wire, and another at the middle. He mounted a tiny mirror
on the works of a watch, so that it revolved at a high velocity, and
observed the reflections of his three sparks in it. The points of the
wire were so arranged that if the sparks were instantaneous, their
reflections would appear in one straight line; but the middle one was
seen to lag behind the others, because it was an instant later. The
electricity had taken a certain time to travel from the ends of the wire
to the middle. This time was found by measuring the amount of lag, and
comparing it with the known velocity of the mirror. Having got the time,
he had only to compare that with the length of half the wire, and he
found that the velocity of electricity was 288,000 miles a second.

Till then, many people had considered the electric discharge to be
instantaneous; but it was afterwards found that its velocity