with the shaft.
THE DE LAVAL TURBINE.
In its earliest form this turbine was a modification of Hero's. The wheel was merely a pipe bent in S form, attached at its centre to a hollow vertical shaft supplied with steam through a stuffing-box at one extremity. The steam blew out tangentially from the ends of the S, causing the shaft to revolve rapidly and work the machinery (usually a cream separator) mounted on it. This motor proved very suitable for dairy work, but was too wasteful of steam to be useful where high power was needed.
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