All About Engines

Steam Turbines 155 It is worth while pausing for a moment to con- sider what these figures mean. If you trundle your 2-foot hoop at the easy pace of 5 miles an hour it covers 6f feet at each turn, and makes nearly one turn a second, nearly fifty-five turns a minute, or 3,405 turns an hour. The 5-foot driving wheel of a locomotive covers 157 feet for every turn, and at sixty miles an hour it will make 5'6 revolutions a second, 336 revolutions a minute, 20,160 revolu- tions an hour, 3,986,880 revolutions in a week of 168 hours. In that time it would have travelled 1,080 miles, or two-fifths of the distance round the world; but if a 6-inch de Laval turbine wheel, rotat- ing at 30,000 revolutions a minute, could be set on a rail, and could keep up its speed, it would cover nearly 9 miles a minute, or 540 miles an hour, or about 90,000 miles in a week of 168 hours. In about forty- six hours it would have gone completely round the earth and arrived again at its starting point. This beats Jules Verne hollow ; for he, riding full tilt on the wings of his imagination, occupied eighty days upon the journey ! Very interesting details of construction follow from the enormous speeds which are obtained in the impulse turbine. From what has been said in Chapter V. about balancing it will be seen that this is of tre- mendous importance where such high speeds are involved. Recalling the formula, —2 which gives b the force produced by a body rotating around a centre (see p. 112), and remembering that even in a disc