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. INDEX Accelerator, 111 Accessories, cost of, 18, 39, 67 Accumulator, 111 Acetylene, 111 Air lever, 37, 43 Ammeter, 55, 111 Ampère, 111 Antioyl soap, 29 Automobile Association, 65 Axles, 111 Backfire, 42, 112 Battery, electric, 39; to test, 55; to adjust, 58; purpose of, 112 Bearings, 112 Belt-transmission, 113 Bevel gear, 113 Big end, 113 Bonnet of car, 33