All About Engines

312 All About Engines Take a piece of coal in your hand and try to realise the hidden power that this black mass contains. It will soil your hands, but that does not matter. You are in the presence of a power so stupendous that it staggers the imagination. There is no need to wonder now that the habits and customs of the civilised world have been altered since James Watt showed how to use the steam engine effectively. For steam is merely the agent which conveys the energy of the coal to the cylinder of the engine, where it drives the piston to and fro, and produces the motion which is used on the railway, in the factory, in the steamship, and the mine. The piston and cylinder, or the turbine, are merely convenient devices for abstracting from the steam the heat which it has absorbed from the furnace. We speak of an engine taking in steam at a certain pressure and exhausting it at another lower pressure, but it is really taking in steam at a high temperature and exhausting it at a lower one. The temperature of exhaust in the case of steam cannot be lower than 32° Fahr, or o° C., because at that temperature water changes into solid ice. The higher temperature is only limited by the ability of the parts of the engine to remain steam tight, by the impossibility of lubricating very highly heated surfaces, and by the difficulty of avoiding loss of heat to surrounding objects, for the loss of heat in this way is roughly proportional to the difference of temperature. More- over, as all gases and vapours expand on heating and exert force by reason of the heat they contain,