Accelerator.—A device, operated by a pedal, for increasing the speed of the engine, either by suspending the controlling action of the governor or opening the throttle.

Accumulator.—An apparatus for storing electricity. See Battery.

Acetylene.—An inflammable gas giving a brilliant light. It is commonly produced by adding water to carbide of calcium.

Ammeter.—An instrument used for measuring the number of ampères in an electrical circuit. See Ampère.

Ampère.—The unit of measure of the quantity of current flowing through an electrical circuit. See Volt.

Axles.—The horizontal shafts or girders by which the weight of carriage is transferred to the road wheels and at the ends of which the wheels revolve. A “live” axle is one which revolves with, and by which the power of the engine is communicated to, the driving-wheels.

Backfire.—A premature explosion of the gaseous mixture in the cylinder. When it occurs while the starting-handle is being turned its effects are distinctly unpleasant to the operator.

Battery.—An arrangement of two or more cells either for the production or the storage of electricity. In the former case it is known as a primary battery; in the latter case it is known as a secondary battery, a storage battery, or an accumulator. See Cell.

Cell, Electrical.—A chemical apparatus for the production or storage of electricity. Two or more cells electrically connected constitute a battery.

Circulating Pump.—The pump which forces the water through the radiator to ensure rapid cooling.

Bearings.—The cradles or surfaces upon which the moving parts of machinery are supported. They always require lubrication.

Belt.—A flexible band sometimes used instead of gearing to transmit the power of the engine to the driving-wheels.

Bevel Gear.—A gear consisting of cog-wheels with bevelled or sloping surfaces.

Big End.—The end of the connecting-rod which grasps the crank. See Connecting-rod.

Brake.—A device for stopping or checking the motion of the car by the application of friction to one or other of the moving parts. A motor-car should have at least three good brakes applicable by the hand and foot of the driver.

Brake Horse-power. See Horse-power.

Cam.—A revolving disc of irregular shape designed to impart a variable motion to some other piece of mechanism—such as the spindle of a valve—in contact with it.

Carbide of Calcium.—A compound of chalk and coke which, when brought into contact with water, gives off the acetylene gas used for lighting.

Carburettor.—The apparatus for regulating the rate of evaporation of the petrol and the proportions of its mixture with air. It consists generally of a float chamber and a mixing-chamber.

Chain.—In motor-cars chains are sometimes used, as in the bicycle, for the transmission of power from one part of the mechanism to another.

Change-speed Gear.—The arrangement of shafts and toothed wheels by which the speed of the road wheels is altered without alterations in the speed of the engine.

Chassis.—The entire framework and mechanism of the car—engines, wheels, axles, &c.—without the body or seating accommodation.

Clutch.—A device for connecting the motive machinery with the driving-wheels at the will of the driver.

Coil.—See Induction Coil.

Commutator.—An appliance for enabling the driver to advance or retard the spark which ignites the mixture in the cylinder.

Compression.—This word in motoring invariably applies to the compression of the gaseous mixture in the cylinder. The efficiency and economy of the motor depend greatly upon the degree of compression at the moment of ignition.

Connecting-rod.—The rod which connects the piston with the crank of the engine.

Cut-out.—A device for diverting the exhaust gases directly into the air instead of compelling them to pass through the silencer. The “cut-out” is seldom used except in racing. It accelerates the engine at the cost of an appalling noise.

Cycle.—See Otto Cycle.

Cylinder.—The cast-iron chamber in which the petrol gas is compressed and exploded and in which the piston travels to and fro.

Densimeter.—An instrument for testing the specific gravity (i.e., the weight as compared with water) of petrol.

Differential Gear.—The gear by which power is communicated to the driving-wheels in such a way that it is taken up automatically by either or both of them according to their respective requirements at the moment.

Dry Cell, or Battery.—A cell, or battery of cells, which works without liquid. Dry cells are generally used for motoring purposes in preference to cells containing solution.

Dynamometer.—An instrument for testing the amount of power expended by mechanical or animal effort. The dynamometer used for testing motors is often called the “pony brake.” See Horse-power.

Electric Ignition.—The ignition of the explosive mixture in the cylinder is effected by an electric spark caused by forcing an electric current through the sparking-plug. The current is obtained (1) from an accumulator or a primary battery, in which case its pressure is raised to the required voltage by passing it through an induction coil; or (2) from a magneto-electrical instrument—which is very much like a dynamo on a small scale, and is driven by the motor.

Exhaust.—The gaseous products of combustion expelled from the cylinder during the scavenging stroke of the piston.

Fan.—A rotary fan, driven by the motor, is often employed to increase the current of air passing through the radiator and thus to assist in cooling the water.

Feed.—The method of conveying the petrol from the main tank to the carburettor. If the tank is higher than the carburettor, the petrol will pass by gravity. Otherwise it must be lifted by pressure. The exhaust is generally employed for this purpose, a hand-pump being fitted to furnish the necessary pressure for starting.

Fly-wheel.—As only one of the four strokes of the motor is a working stroke, a heavy fly-wheel is necessary to carry the piston through its cycle and promote easy running.

Frame.—The structure which carries the machinery of the car.

Garage.—A stable for motor-cars.

Gears.—See Bevel Gear, Change-speed Gear, Differential Gear.

Governor.—The appliance which automatically regulates the speed of the engine, usually by checking the volume of mixture admitted to the cylinder.

Gradient.—The inclination or slope of a road.

Gravity Feed.—See Feed.

Horse-power.—Boulton and Watt calculated that a London dray-horse was capable of work equivalent to lifting 33,000 lb. one foot high in one minute, and this task—technically described as 33,000 foot-lb. per minute—has been accepted as the “unit of horse-power” for the measurement of mechanical work. The power of a petrol motor depends upon many factors—diameter of cylinder, speed of working, quality of mixture, compression, cooling-surfaces, &c.—some of which vary from moment to moment in practice. The only real means of measuring it is by the dynamometer or pony-brake, which records the power actually available for useful work. Horse-power so measured is called brake horse-power. For purposes of competitions the Royal Automobile Club use the following formula for rating the comparative power of petrol motors. Multiply the diameter of the cylinder in inches by itself and again by the number of cylinders. The product divided by 2¼ is the nominal horse-power.

Ignition.—See Electric Ignition.

Induction-Coil.—An apparatus for intensifying the pressure of the electric current. Used in motor practice as a part of the system of high-tension ignition.

Inspection Pit.—A pit or well, generally situated in or near the garage, to facilitate the examination and repair of the chassis of a car.

Licences.—Licences must be taken out (1) by the owner of a car. This licence costs from two to five guineas according to the weight of the car; and (2) the driver of a car. This costs five shillings only.

Limousine.—A large covered car.

Live Axle.—See Axles.

Lubrication.—The application of oil, grease or other substances suitable for the reduction of friction between sliding surfaces.

Mixture.—The explosive charge of petrol and air admitted to the cylinder. See Carburettor.

Otto Cycle.—Nearly all petrol motor-car engines work upon the system invented by Otto in 1876 and known as the Otto Cycle. The cycle consists of the successive operation of induction, compression, explosion and scavenging, there being thus only one working stroke in four—that is in every two revolutions of the fly-wheel.

Over-Heating.—An accident generally resulting from deficient water circulation or insufficient lubrication. Unless speedily remedied it may result in serious damage to the motor. See Seizing.

Panne.—A French word, meaning “a breakdown.”

Petrol.—A highly inflammable and volatile spirit distilled from petroleum. Seven pints of petrol weigh about as much as ten pints of water. Its vapour is heavier than air.

Piston.—The disc which slides up and down in the cylinder, and communicates the force of the explosion to the connecting-rod and crank.

Piston Rings.—Cast-iron split rings, fitted in grooves round the piston to make a gas-tight joint between the piston and the walls of the cylinder.

Pony-brake.—See Dynamometer.

Pressure Feed.—See Feed.

Radiator.—A device for cooling the water circulating round the cylinders by exposing it to a large surface in contact with free air.

Side-Slip.—An accident liable to occur on greasy roads, but of less frequent occurrence since the introduction of “non-skidding” devices.

Silencer.—A box or chamber designed to reduce the noise caused by the escape of the exhaust gases.

Spares.—Duplicate parts of machinery carried in case of a breakdown.

Sparking-plug.—A porcelain plug, carrying the electric wires, which is screwed into the combustion chamber of the cylinder. At the end of the plug within the cylinder are platinum points in connection with the wires. The current jumping from point to point makes the spark which fires the charge.

Throttle.—The control of the volume of mixture supplied to the engine.

Timing-gear.—The gear which controls the times at which the valves of the engine open and close, and at which the charge is fired.

Torque.—The twisting effort of rotation.

Universal Joint.—A flexible joint which permits the transmission of power from one shaft to another in any direction.

Volt.—The unit of measure of electrical pressure. It is the measure of the quality of the flow as compared with ampere which is the measure of quantity.