In engines for pumping or for blowing air it is not essential to drive a revolving shaft, and in many forms the reciprocating motion of the steam piston is applied directly to produce the reciprocating motion of the pump-piston or plunger. Often an inverted vertical triple expansion engine is used, resembling the usual marine form, with a pump-plunger under each of the three cylinders. Most of the power is directly transmitted from the piston to the pump, but there is an auxiliary crank for each cylinder, driving a shaft with a flywheel to equalize the effort on the pump.
A familiar example of direct-acting pump is the small donkey pump used for feeding boilers, in which the steam-piston and pump-plunger are on one and the same rod. In some of these pumps a rotative element is introduced, partly to secure steadiness of running and partly for convenience in working the valves. But many pumps of this class are entirely non-rotative, and in such cases the steam is generally admitted throughout the stroke with out expansion. The valve may be actuated by tappets from the piston-rod. In some cases a tappet worked by the piston as it reaches each end of its stroke throws over an auxiliary steam valve, which admits steam to one or other side of an auxiliary piston carrying the main slide-valve.
Hall's "pulsometer" is a peculiar pumping engine without cylinder or piston, which may be regarded as the modern representative of the engine of Savery. The sectional view, fig. shows its principal parts. There are two chambers, A, A', nar rowing towards the top, where the steam-pipe B enters. A ball vaive C allows steam to pass into one of the chambers and closes the other. Steam entering (say) the right-hand chamber forces water out of it past the clack-valve V into a delivery passage D, which is connected with an air-vessel. When the water level in A sinks so far that steam begins to blow through the delivery pas sage, the water and steam are disturbed and so brought into inti mate contact, the steam in A condenses, and a partial vacuum is formed. This causes the ball-valve C to rock over and close the top of A, while water rises from the suction-pipe E to fill that chamber. At the same time steam begins to enter the other cham ber A', discharging water from it, and the same series of actions is repeated in each chamber alternately. While the water is being
driven out there is comparatively little condensation of steam, partly because the shape of the vessel does not promote the formation of eddies, and partly because there is a cushion of air between the steam and the water. Near the top of each chamber is a small air-valve opening inwards, which allows a little air to enter each time a vac uum is formed. When any steam is con densed, the air mixed with it remains on the cold surface and forms a non-conduct ing layer. The pulsomcter is, of course, far from efficient as a thermodynamic en gine, but its adaptability to situations where other steam-pumps cannot easily be applied, and the simplicity of its work ing parts, sometimes make it practically useful.
The early steamers were fitted with paddle-wheels, and the engines used to drive them were for the most part modified beam engines. Bell's "Comet" was driven by a species of inverted beam engine, and another form of inverted beam, known as the side-lever engine, was for long a favourite with marine engineers. An old form of direct-acting paddle-engine was the steeple engine, in which the cylinder was set vertically below the crank. Two piston-rods projected through the top of the cylinder, one on each side of the shaft and of the crank. They were united by a: cross-head sliding in vertical guides, and from this a return-connecting-rod led to the crank. Most modern paddle-wheel engines are direct-acting engines of the ordi nary connecting-rod type, with the cylinders fixed on an inclined bed and the guides sloping up towards the shaft.
When the screw-propeller began to take the place of paddle wheels in ocean steamers, the increased speed which it required was at first obtained by using spur-wheel gearing in conjunction with one of the forms of engines then usual in paddle steamers. Types of engine better suited to the screw were introduced later, and were driven fast enough to be connected directly to the screw shaft. After passing through various forms, now obsolete, the marine engine settled into one which is now universal in all screw steamships that are not driven by steam turbine.