In the De Laval turbine as now constructed the steam is blown from stationary nozzles against vanes mounted on a revolving wheel. Fig. 36 shows the nozzles and a turbine wheel. The wheel is made as a solid disc, to the circumference of which the vanes are dovetailed separately in a single row. Each vane is of curved section, the concave side directed towards the nozzles, which, as will be gathered from the "transparent" specimen on the right of our illustration, gradually expand towards the mouth. This is to allow the expansion of the steam, and a consequent gain of velocity. As it issues, each molecule strikes against the concave face of a vane, and, while changing its direction, is robbed of its kinetic energy, which passes to the wheel. To turn once more to a stone-throwing comparison, it is as if a boy were pelting the wheel with an enormous number of tiny stones. Now, escaping high-pressure steam moves very fast indeed. To give figures, if it enters the small end of a De Laval nozzle at 200 lbs. per square inch, it will leave the big end at a velocity of 48 miles per _minute_--that is, at a speed which would take it right round the world in 8-1/2 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.