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.