r relates. The air opens a valve and puts the reservoir
in connection with a piston operating the points or signal-arm, as the
case may be. This movement having been performed, another valve in the
reservoir is opened, and air passes back through a second pipe to the
signal-box, where it opens a third valve controlling a piston which
completes the movement of the lever, so showing the signalman that the
operation is complete. With compressed air, as with electricity, a
mechanical locking-frame is of course used.

AUTOMATIC SIGNALLING.

To reduce expense, and increase the running speed on lines where the
sections are short, the train is sometimes made to act as its own
signalman. The rails of each section are all bonded together so as to be
in metallic contact, and each section is insulated from the two
neighbouring sections. At the further end of a section is installed an
electric battery, connected to the rails, which lead the current back to
a magnet operating a signal stationed some distance back on the
preceding section. As long as current flows the signal is held at "All
right." When a train enters the section the wheels and axles
short-circuit the current, so that it does not reach the signal magnet,
and the signal rises to "danger," and stays there until the last pair of
wheels has passed out of the section. Should the current fail or a
vehicle break loose and remain on the section, the same thing would
happen.

The human element can thus be practically eliminated from signalling. To
make things absolutely safe, a train should have positive control over a
train following, to prevent the driver overrunning the signals. On
electric railways this has been effected by means of contacts working
in combination with the signals, which either cut the current off from
the section preceding that on which a train may be, or raise a trigger
to strike an arm on the train following and apply its brakes.

Chapter XII.

OPTICS.

Lenses--The image cast by a convex lens--Focus--Relative position
of object and lens--Correction of lenses for colour--Spherical
aberration--Distortion of image--The human eye--The use of
spectacles--The blind spot.

Light is a third form of that energy of which we have already treated
two manifestations--heat and electricity. The distinguishing
characteristic of ether light-waves is their extreme rapidity of
vibration, which has been calculated to range from 700 bi