or go slowly in crowded places, or fast in
quiet ones, and he can even reverse the motor and make the car go
backwards, a thing that neither the driver of the horse-car nor the
gripman of the cable-car can do.

Perhaps it is not quite clear how so many motors can work from a single
line. As a rule electric railways are provided with but a single wire
from which the motors obtain their supply of current, and this system
has come to be called the single trolley system in distinction to the
double trolley, or double wire system, which was tried in the early days
but has been abandoned in all but one or two places. The single wire
hung over the centre of the track carries the current out from the
station where the dynamos are placed, and the rails and the earth carry
it back to the dynamos after it has passed through the motors and has
done its work. The trolley wire is kept constantly charged with
electricity, which the dynamos at the power station pump into it, much
as if they were pumping-engines forcing water into a long pipe. If any
connection is made, by means of an electrical conductor, between the
trolley-wire and the ground, the current will flow down into the ground.
The only connections made with it are those made by the cars, and then
the current has to pass through the motors and turn the wheels.

The trolley-wire has to be carefully put up so as to be just the right
height, and exactly in the middle of the track. It must be properly
insulated so as to prevent the escape of the current down the poles or
along the suspension wires, so at every point where it is attached to a
pole or a suspension wire it is hung from an insulator of some material
that will not conduct electricity. Every here and there you will notice
that heavy electric wires or cables are connected with the
trolley-wires. These-wires are called "feeders"; they are run out from
the dynamos at the power-house, and connect on to the trolley-wire to
force fresh supplies of current into it. When an electric current
travels along a wire it loses a certain amount of power by reason of the
resistance or electrical friction of the wire itself, so in order to
keep the supply of current up to the proper pitch required for working
the motors at all points of the line, these "feeders" are run out from
the power-house, and they literally feed the trolley-wire with the
current that the cars are always demanding from it.

It is often said in the newspapers th