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cc that on the opposite side; de the connecting-rod, joined at one end d to the piston, and at the other e to the crank on the “propeller" shaft i f and g show the vibration of the connecting-rod in various points of the revolution of the crank. By this arrangement of piston and crank, it will
be observed that the steam does not press on the whole surface of the piston, but only in the annular ring a a, between the outside of the trunk b, fig. 173, and the interior of the cylinder in which the piston moves.
Governors are usually applied to screw-engines in vessels of light draught, in order to regulate the speed. They are of the usual construction, although many new schemes have been recently introduced; for notices of these we refer the reader to the pages of the Artisan, more particularly the numbers for 1853. The necessity for a regulator of the speed of the screw-engine will be obvious, in considering that as the screw is placed at the extremity of the vessel, as it pitches in a heavy sea, the screw would revolve in the air; and as the load would thus be taken from the engine, it would revolve at too great a speed.
We must not omit the mention of an entirely new and very good form of engine adapted for working the screw-propeller, lately designed by Mr. John C. Bothams, engineer, of Salisbury, which differs from the ordinary marine engines in not being a pair of engines of the same description, with
all the working parts in duplicate, but a combination of two engines of different form : the one A an ordinary oscillating engine, having an oscillating motion given to it by being attached to the piston p of the pendulous
engine B, by the hollow shaft c, which conveys steam to the oscillating cylinder A ; the centre of gravity of the moving parts, the pendulous piston P, and the oscillating cylinder A being fixed as near as possible the required distance below the point of suspension c, according to the laws of pendulous motion.
The piston-rods r of the oscillating cylinder a have motion in two directions: a perpendicular motion derived from the cylinder A, and a horizontal motion from the pendulous piston P; the result being a circular motion to drive the screw-shaft s by means of a single crank.
Fig. 174 is a side view of the two engines, and fig. 175 a section through the pendulous engine B. The steam enters by the pipe E, and passes out by the pipes F F F to the condenser.
Fig. 176 represents an equilibrium surface condenser, and the pump to
supply it with cold water : the former consists of a case BB sufficiently strong to resist the pressure of the atmosphere, and is divided into two separate spaces by the plates CD; through the upper plate c a number of thin metal tubes are passed, and are screwed by their lower ends into the plate D; the steam to be condensed is admitted at a into the steam-space s, and flows among or between the tubes, and is condensed by contact with the outer surface of the tubes, which are kept cold by a constant supply of water flowing over their inside surface. Pis a pump to supply the condenser with cold water, which enters the pump at E above the piston, is lifted
into the upper part of the condenser w, flows down through the tubes into the lower space w, then is drawn into the lower part of the pump, and by the down-stroke of the piston is forced out at the opening F. No water being admitted to the condenser except when an equal quantity is pumped out at the same time, the tubes are relieved of the pressure of the atmosphere on their inside surface. The condensed steam flowing down the tubes on to the lower plates D, flows into the space 7, and is pumped out by the feed-pump into the boiler.
In order to relieve the tubes from all inequality of pressure on their inner and outer surfaces, a pipe k, leading from the upper water-space w, down into the reservoir i, at the bottom of the steam-space s, acts as a trap; the water in a derived from the condensed steam being above the bottom
of the pipe K, prevents a communication between the water-space ww and the steam-space s, as long as those spaces are in equilibrium, or nearly so.
The tubes, therefore, being relieved of the atmospheric pressure, are only employed to separate the steam from the salt water used for condensing, and need not be formed of metal of a greater thickness than llb. weight per square foot, and can also be of any form, corrugated as in fig. 177, to diminish the number of tubes and joints, the amount of surface being the same.
fig. 177. The thinner the metal which can be used, the less the amount of surface required to condense a given quantity of steam. The pressures on the upper and under sides of the piston being always equal, the pump can be worked by a small amount of power, in the case of large marine engines, by the auxiliary engine used to fill the boilers.
The use of brine-pumps, refrigerator, salinometer, &c., is dispensed with, and also the waste of about one-fourth of the whole quantity of feedwater supplied to the boiler, after being raised to the boiling point, whereby a considerable saving of fuel is effected; and it must be borne in mind that for every ton of fuel saved a ton of freightage is gained.
This invention must ultimately prove of great value; and in bringing it to the notice of engineers, we are thereby rendering them good service.
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