All About Engines

324 All About Engines test may cause them to burst. A safer plan, there- fore, is to fix a pulley to the other end of the shaft to take the brake. When one comes to hundreds and thousands of horse-power it is clearly impossible to absorb the energy by friction in this way. Thus if the engine were only of 1,000 horse-power it would raise more than 2 cwt. of water from the freezing point to the boiling point every minute, or it would bring a ton of water from the freezing to the boiling point in nine and a half minutes. But though solid friction is out of the question, liquid friction can be used, and a Froude’s hydraulic brake is occasionally employed. There are, however, other means of testing. Thus a pump is designed to lift a certain amount of water a certain height in a certain time with a certain consumption of coal ; and the coal used and the water pumped can be measured without much diffi- culty. Again, a ship is required to make a certain speed on a certain coal consumption, and this can be tested. Further, an engine for an electric light station is required to produce a certain quantity of elec- tricity for a given consumption of coal or steam, and electrical energy is more easily measured than mechanical energy ; so that the power of electric light engines is generally given in “ kilowatts ” rather than in “ horse-power.” As a kilowatt is 1,000 watts, and 746 watts correspond to 1 horse-power, the latter can easily be calculated from the electrical output. The largest and most powerful engines in the world are found in electric light stations, on steam-