smooth and continuous action is secured. These engines have
been extensively applied, on a large scale, to raise water for the
supply of towns and to force oil through “pipe-lines” in the
United States. In the larger sizes they are made compound,
each high-pressure cylinder having a low-pressure cylinder
tandem with it on the same rod. To allow of expansive working 
Fig. 49.
a device is added which compensates for the inequality of effort
resulting from an early cut-off. A
cross-head A (fig. 40) fixed to each
of the piston-rods is connected to
the piston-rods of a pair of oscillating
cylinders B, B, which contain
water and communicate with a
reservoir full of air compressed to
a pressure of about 300 ℔ per square
inch. When the stroke (which
takes place in the direction of the
arrow) begins the pistons are at first
forced in, and work is at first done
by the main piston-rod, through the compensating cylinders
B, B, on the compressed air in the reservoir. This continues
until the cross-head has advanced so that the cylinders stand
at right angles to the line of stroke. Then for the remainder
of the stroke the compensating cylinders assist in driving the
main piston, and the compressed air gives out the energy which
it stored in the earlier portion. The volume of the air reservoir
is so much greater than the volume of the cylinders, B, B, that
the air pressure remains nearly constant throughout the stroke.
Any leakage from the cylinder or reservoir is made good by a
small pump which the engine drives.
99. Pulsometer.—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. 50, shows its principal parts. There
are two chambers A, A′, narrowing
towards the top, where the steam-pipe
B enters. A ball-valve C allows steam to 
Fig. 50. Pulsometer.
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 passage, the water
and steam are disturbed and so brought
into intimate contact, the steam in A
is condensed, 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 chamber A′, discharging
water from it, and the same series of actions is repeated in
either 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 vacuum is formed. When any steam is condensed,
the air mixed with it remains on the cold surface and forms a
non-conducting layer. The pulsometer is, of course, far from
efficient as a thermodynamic engine, but its suitability for
situations where other steam-pumps cannot be used, and the
extreme simplicity of its working parts, make it valuable in
certain cases.
100. Rotary Engines.—From the earliest days of the rotative engine attempts have been made to avoid the intermittent reciprocating motion which an ordinary piston engine first produces and then converts into motion of rotation. Murdoch, the contemporary of Watt, proposed an engine consisting of a pair of spur-wheels gearing with one another in a chamber through which steam passed by being carried round the outer sides of the wheels in the spaces between successive teeth.
In Dudgeon’s wheel engine the steam was admitted by ports in side-plates into the clearance space behind teeth in gear with one another, just after they had passed the line of centres. From that point to the end of the arc of contact the clearance space increased in volume; and it was therefore possible, by stopping the admission of steam at an intermediate point, to work expansively. The difficulty of maintaining steam-tight connexion between the teeth and the side-plates on which the faces of the wheels slide is obvious; and the same difficulty has prevented the success of many other forms of rotary engine. These have been devised in immense variety, in many cases, it would seem, with the idea that a distinct mechanical advantage was to be secured by avoiding the reciprocating motion of a piston. In point of fact, however, very few forms entirely escape having pieces with reciprocating motion. In all rotary engines, with the exception of steam turbines—where work is done not by pressure but by the kinetic impulse of steam—there are steam chambers which alternately expand and contract in volume, and this action usually takes place through a more or less veiled reciprocation of working parts. So long as engines work at a moderate speed there is little advantage in avoiding reciprocation; the alternate starting and stopping of piston and piston-rod does not affect materially the frictional efficiency, throws no deleterious strain on the joints, and need not disturb the equilibrium of the machine as a whole. The case is different when very high speeds are concerned; it is then desirable as far as possible to limit the amount of reciprocating motion and to reduce the masses that partake in it.
101. Types of Marine Engines.—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. In the side-lever engine the cylinder was vertical, and the piston-rod projected through the top. From a crosshead on the rod a pair of links, one on each side of the cylinder, led down to the ends of a pair of horizontal beams or levers below, which oscillated about a fixed gudgeon at or near the middle of their length. The two levers were joined at their other ends by a cross-tail, from which a connecting-rod was taken to the crank above. The side-lever engine is now obsolete. In American practice, engines of the beam type, with a braced-beam supported on A frames above the deck, are still common in river-steamers and coasters. 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 crosshead sliding in vertical guides, and from this a return-connecting-rod led to the crank. Modern paddle-wheel engines are usually of one of the following types. (1) In oscillating cylinder engines the cylinders are set under the crank-shaft, and the piston-rods are directly connected to the cranks. The cylinders are supported on trunnions which give them the necessary freedom of oscillation to follow the movement of the crank. Steam is admitted through the trunnions to slide-valves on the sides of the cylinders. In some instances the mean position of the cylinders is inclined instead of vertical; and oscillating engines have been arranged with one cylinder before and another behind the shaft, both pistons working on one crank. The oscillating cylinder type is best adapted for what would now be considered comparatively low pressures of steam. (2) Diagonal engines are direct-acting engines of the ordinary 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 supplied by using spur-wheel gearing in conjunction with one of the forms of engines then usual in paddle steamers.
