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.