speed which would take it right round the world in 8½ hours! The wheel itself would not move at more than about one-third of this speed as a maximum.7 But even so, it may make as many as 30,000 revolutions per minute. A mechanical difficulty is now encountered—namely, that arising from vibration. No matter how carefully the turbine wheel may be balanced, it is practically impossible to make its centre of gravity coincide exactly with the central point of the shaft; in other words, the wheel will be a bit—perhaps only a tiny fraction of an ounce—heavier on one side than the other. This want of truth causes vibration, which, at the high speed mentioned, would cause the shaft to knock the bearings in which it revolves to pieces, if—and this is the point—those bearings were close to the wheel M. de Laval mounted the wheel on a shaft long enough between the bearings to "whip," or bend a little, and the difficulty was surmounted.
The normal speed of the turbine wheel is too high for direct driving of some machinery, so it is reduced by means of gearing. To dynamos, pumps, and air-fans it is often coupled direct.
THE PARSONS TURBINE.
At the grand naval review held in 1897 in honour of Queen Victoria's diamond jubilee, one of the most noteworthy sights was the little Turbinia of 44½ tons burthen, which darted about among the floating forts at a speed much surpassing that of the fastest "destroyer." Inside the nimble little craft were engines developing 2,000 horse power, without any of the clank and vibration which usually reigns in the engine-room of a high-speed vessel. The Turbinia was the first turbine-driven boat, and as such, even apart from her extraordinary pace, she attracted great attention.