le has travelled to the receiving end
before sending another, a series of waves may be sent after each other
in rapid succession. These waves, encroaching upon each other, will
coalesce at their bases; but if the crests remain separate, the delicate
decipherer at the other end will take cognisance of them and make them
known to the eye as the distinct signals of the message.

The mirror galvanometer is at once beautifully simple and exquisitely
scientific. It consists of a very long fine coil of silk-covered copper
wire, and in the heart of the coil, within a little air-chamber, a small
round mirror, having four tiny magnets cemented to its back, is hung, by
a single fibre of floss silk no thicker than a spider's line. The mirror
is of film glass silvered, the magnets of hair-spring, and both together
sometimes weigh only one-tenth of a grain. A beam of light is thrown
from a lamp upon the mirror, and reflected by it upon a white screen or
scale a few feet distant, where it forms a bright spot of light.

When there is no current on the instrument, the spot of light remains
stationary at the zero position on the screen; but the instant a
current traverses the long wire of the coil, the suspended magnets twist
themselves horizontally out of their former position, the mirror is of
course inclined with them, and the beam of light is deflected along the
screen to one side or the other, according to the nature of the current.
If a POSITIVE current--that is to say, a current from the copper pole
of the battery--gives a deflection to the RIGHT of zero, a NEGATIVE
current, or a current from the zinc pole of the battery, will give a
deflection to the left of zero, and VICE VERSA.

The air in the little chamber surrounding the mirror is compressed at
will, so as to act like a cushion, and 'deaden' the movements of the
mirror. The needle is thus prevented from idly swinging about at each
deflection, and the separate signals are rendered abrupt and 'dead
beat,' as it is called.

At a receiving station the current coming in from the cable has simply
to be passed through the coil of the 'speaker' before it is sent into
the ground, and the wandering light spot on the screen faithfully
represents all its variations to the clerk, who, looking on, interprets
these, and cries out the message word by word.

The small weight of the mirror and magnets which form the moving part of
this instrument, and the range to which the minute motions