In reciprocating--that is, cylinder--engines steam is admitted into a chamber and the door shut behind it, as it were. As it struggles to expand, it forces out one of the confining walls--that is, the piston--and presently the door opens again, and allows it to escape when it has done its work. In Hero's toy the impact of the issuing molecules against other molecules that have already emerged from the pipes was used. One may compare the reaction to that exerted by a thrown stone on the thrower. If the thrower is standing on skates, the reaction of the stone will cause him to glide backwards, just as if he had pushed off from some fixed object. In the case of the _reaction_--namely, the Hero-type--turbine the nozzle from which the steam or water issues moves, along with bodies to which it may be attached. In _action_ turbines steam is led through fixed nozzles or steam-ways, and the momentum of the steam is brought to bear on the surfaces of movable bodies connected 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. [Illustration: FIG. 36.--The wheel and nozzles of a De Laval turbine.]