All About Engines

58 All About Engines for every temperature a corresponding pressure. The power of a boiler is measured by the amount of water it will convert into steam at a particular tempera- ture or pressure every hour, and it may vary from a thousand or two to thirty or forty thousand pounds per hour! Now the first essential of all boilers is that they shall be capable of withstanding the pressure of the steam they produce, and the second is that the heat shall be conveyed rapidly and without loss to the water. With every pound of coal yielding 14,000 units of heat during combustion, and every unit of heat transformed into work capable of doing 778 ft.-lb. of work or lifting over a third of a ton 1 foot from the ground, there is need of strength if acci- dents are to be avoided. Professor Thurstan calcu- lated that, at a pressure of 100 lb. on the square inch, there is sufficient energy inside an ordinary cylindrical boiler to hurl it 3J miles in the air. Yet modern boilers are working every day at 200 lb. and 225 lb. on the square inch, and men are moving freely about among them, secure in their confidence that the design and workmanship are equal to any test which could be applied. To a person with any imagination it is a queer sensation to walk over the top of one of these reservoirs of energy and to realise that within a few seconds you might find yourself 15,000 feet or thereabouts above the ground. The best shape for resisting internal pressure is a sphere, because a sphere has a smaller area of sur- face for a given volume, or, conversely, a larger