While this takes long to describe, the motion is faster than the eye can follow, and the “phut, phut” noise of the exhaust sounds like the tattoo of a drum.  Almost every gasoline motor vehicle carries its own electric plant, either a set of batteries or more commonly a little magneto dynamo, which is run by the shaft of the motor.  Electricity is used to make the spark that explodes the gas at just the right moment in the cylinders.  All this is automatic, though sometimes the driver has to resort to the persuasive qualities of a monkey-wrench and an oil-can.

The exploding gas creates great heat, and unless something is done to cool the cylinders they get so hot that the gas is ignited by the heat of the metal.  Some motors are cooled by a stream of water which, flowing round the cylinders and through coils of pipe, is blown upon by the breeze made by the movement of the vehicle.  Others are kept cool by a revolving fan geared to the driving-shaft, which blows on the cylinders; while still others—­small motors used on motor bicycles, generally—­have wide ridges or projections on the outside of the cylinders to catch the wind as the machine rushes along.

The inventors of the gasoline motor vehicles had many difficulties to overcome that did not trouble those who had to deal with steam.  For instance, the gasoline motor cannot be started as easily as a steam-engine.  It is necessary to make the driving-shaft revolve a few times by hand in order to start the cylinders working in their proper order.  Therefore, the motor of a gasoline machine goes all the time, even when the vehicle is at rest.  Friction clutches are used by which the driving-shaft and the axles can be connected or disconnected at the will of the driver, so that the vehicle can stand while the motor is running; friction clutches are used also to throw in gears of different sizes to increase or decrease the speed of the vehicle, as well as to drive backward.

[Illustration:  AN AUTOMOBILE BUCKBOARD]

The early gasoline automobiles sounded, when moving, like an artillery company coming full tilt down a badly paved street.  The exhausted gas coughed resoundingly, the gears groaned and shrieked loudly when improperly lubricated, and the whole machine rattled like a runaway tin-peddler.  Ingenious mufflers have subdued the sputtering exhaust, the gears are made to run in oil or are so carefully cut as to mesh perfectly, rubber tires deaden the pounding of the wheels, and carefully designed frames take up the jar.

Steam and gasoline vehicles can be used to travel long distances from the cities, for water can be had and gasoline bought almost anywhere; but electric automobiles, driven by the third of the three powers used for self-propelled vehicles, must keep within easy reach of the charging stations.

Just as the perfection of the gasoline motor spurred on the inventors to adapt the steam-engine for use in automobiles, so the inventors of the storage battery, which is the heart of an electric carriage, were stirred up to make electric propulsion practical.