f a key by the signalman
at the further end of the block into which the train is to run, and by
the turning of a handle by the signalman who requires to withdraw a
staff. The magnet, being energized, is able to lift a mechanical lock,
and permits the withdrawal of a staff. In its passage through the
instrument the staff revolves a number of iron discs, which in turn
raise or lower a switch controlling the electrical connections. This
causes the electric currents actuating the electro-magnet to oppose
each other, the magnetism to cease, and the lock to fall back,
preventing another staff being withdrawn. It will naturally be asked,
"How is the electrical system restored?" We have said that there were a
number of staffs in each instrument--in other words, a given number of
staffs, usually twenty, is assigned to the section. Assume that there
are ten in each instrument, and that the switch in each is in its lower
position. Now withdraw a staff, and one instrument has an odd, the other
an even, number of staffs, and similarly one switch is raised while the
other remains lowered, therefore the electrical circuit is "out of
phase"--that is, the currents in the magnets of each staff instrument
are opposed to one another, and cannot release the lock. The staff
travels through the section and is placed in the instrument at the other
end, bringing the number of staffs to eleven--an odd number, and, what
is more important, _raising_ the switch. Both switches are now raised,
consequently the electric currents will support each other, so that a
staff may be withdrawn. Briefly, then, when there is an odd number of
staffs in one instrument and an even number in the other, as when a
staff is in use, the signalmen are unable to obtain a staff, and
consequently cannot give authority for a train to enter the section; but
when there is either an odd or an even number of staffs in each
instrument a staff may be withdrawn at either end on the co-operation of
the signalmen.

We may add that, where two instruments are in the same signal-box, one
for working to the box in advance, the other to the rear, it is arranged
that the staffs pertaining to one section shall not fit the instrument
for the other, and must be of different colours. This prevents the
driver accidentally accepting a staff belonging to one section as
authority to travel over the other.

INTERLOCKING.

The remarks made on the interlocking of points and signals on double
l