by Messrs. Hawthorn, of Newcastle. The low-pressure cylinders are each 35 inches diameter and 7 feet stroke, and the high-pressure cylinders are each of 20 inches diameter and 5 feet 3 inches stroke. The pressure of steam in the boilers is 28 lbs. per square inch, and it may be cut off at ths, ths, or ths of the small cylinder. The ultimate pressure of the steam in the large cylinder is about 3 lbs. Each engine is separate, and has its own fly wheel 20 feet 8 inches diameter and weighing 16 tons. Each engine makes about 15 revolutions per minute and exerts 62:45 actual horses-power, with a consumption of 3.63 lbs. of coal per actual horsepower per hour when using two boilers, and 3'34 lbs. when using three. The pumps, being placed at half the radius of the beam, have a stroke of 3 feet and a cubical capacity of 8 cubic feet. The air vessel is 5 feet diameter and 18 feet high, and the supply of air is maintained within it by means of small air valves on the suction side of the pumps. The suction pipe is 21 inches diameter, and the delivery pipe 15 inches diameter. The total cost of these waterworks was about 80,000l., of which the engines, boilers, and fittings cost 7,000l., or less than 1th of the whole. A better performance than the foregoing has been obtained in many pumping engines of the single cylinder type, and such engines are simpler. It would have been better if the pressure of the steam in the Altona engines had been twice as great as it is.

10

VARIOUS FORMS OF MARINE ENGINES.

2. What species of paddle engine do you consider to be the best?

A. The oscillating engine.

2. Will you explain the grounds of that preference?

A. The engine occupies little space, consists of few parts, is easily accessible for repairs, and may be both light and strong at the same time. In the case of large engines the crank in the intermediate shaft was formerly a disadvantage, as it was difficult to obtain such a forging quite sound. But by forging it in three cranked flat bars, which were then laid together and welded into a square shaft, a sound forging was got. The cranked bars were rounded a little on the sides which were welded, to allow the scoriæ to escape during that operation. It is important in so large a forging not to let the fire be too fierce, else the surface of the iron will be burnt before the heart is brought to a welding heat. In some cases

Objections to Oscillating Engines Considered.

251 in oscillating engines the air pump has been wrought by an eccentric, and that may at any time be done where doubt of obtaining a sound intermediate shaft is entertained. But the precaution must be taken to make the eccentric very wide, so as to distribute the pressure over a large surface, else the bearing will be apt to heat, and an eccentric is more clumsy than a crank. A crank obtained by bending the shaft will do very well. But there is no longer any difficulty in making cranks of any desired form perfectly sound. 2. Have not objections been brought against the oscillating engine?

A. In common with every other improvement, the oscillating engine, at the time of its introduction, encountered much opposition. The cylinder, it was said, would become oval, the trunnion bearings would be liable to heat and the trunnion joints to leak, the strain upon the trunnions would be apt to bend in or bend out the sides of the cylinder; and the circumstance of the cylinder being fixed across its centre, while the shaft requires to accommodate itself to the working of the ship, might, it was thought, be the occasion of such a strain upon the trunnions as would either break them or bend the piston rod. It is a sufficient reply to these objections to say that they are all hypothetical, and that none of them in practice have been found to exist-to such an extent, at least, as to occasion any inconvenience. But it is not difficult to show that they are altogether unsubstantial, even without a recourse to the disproofs afforded by experience.

Q. Is there not a liability in the cylinder to become oval from the strain thrown on it by the piston ?

A. There is, no doubt, a tendency in oscillating engines for the cylinder and the stuffing-box to become oval. But after a number of years' wear it is found that the amount of ellipticity is less than that which is found to exist in the cylinders of side lever engines after a similar trial. The resistance opposed by friction to the oscillation of the cylinder is so small, that a man is capable of moving a large cylinder with one hand; whereas in the side lever engine, if the parallel motion be in the least untrue, which is, at some time or other, an almost inevitable condition, the piston is pushed with great force against the side of the cylinder, whereby a large amount of wear and friction is occasioned. The trunnion bearings, instead of being liable to heat like other journals, are kept down to the temperature of the steam by the flow of steam passing through them; and the trunnion packings are not liable to

leak when the packings, before being introduced, are squeezed in a cylindrical mould.

2. Might not the eduction trunnions be immersed in water?

A. In some cases a hollow, or lantern brass, about one-third or one-fourth the length of the packing space, and supplied with steam or water by a pipe, has been introduced in the middle of the packing, so that if there were any leakage through the trunnion, it would be a leakage of steam or water, which would not vitiate the vacuum. But in ordinary cases this device is not necessary, and it is not commonly employed. It is clear that there can be no buckling of the sides of the cylinder by the strain upon the trunnions, if the cylinder be made strong enough, and in cylinders of the ordinary thickness such an action has never been experienced. Nor is it the fact, that the intermediate shaft of steam vessels, to which part alone the motion is communicated by the engine, requires to adapt itself to the altering forms of the vessel, as the engine and intermediate shaft are rigidly connected, although the paddle shafts require to be capable of such an adaptation. Even if this objection existed, however, it could easily be met by making the crank pin of the ball and socket fashion, which would permit the position of the intermediate shaft, relatively with that of the cylinder, to be slightly changed, without throwing an undue strain upon any of the working parts.

Q. Is the trunk engine inferior to the oscillating?

A. A very elegant and efficient arrangement of trunk engine suitable for paddle vessels has sometimes been employed by Messrs. Rennie, of which all the parts resemble those of Penn's oscillating engine except that the cylinders are stationary instead of being movable; and a round trunk or pipe set upon the piston, and moving steam-tight through the cylinder cover, enables the connecting rod which is fixed to the piston to vibrate within it to the requisite extent. But the vice of all trunk engines is that they are necessarily more wasteful of steam, as the large mass of metal entering into the composition of the trunk, moving as it does alternately into the atmosphere and the steam, must cool and condense a part of the steam. The radiation of heat from the interior of the trunk will have the same operation, though the loss from this cause might probably be reduced by lining the trunk with wood. Q. What species of screw engine do you consider the best? A. If the engine has to be kept below the water line I am inclined to give the preference to the variety of the horizontal

Best Forms of Screw Engines.

In

253 steeple engine, first used in H.M.S. 'Amphion,' and of which a representation has already been given in fig. 57, which represents one of the engines of the French war steamer 'Friedland.' this engine the cylinders lie on their sides, and they are placed near the side of the vessel with their mouths pointing towards the keel. From each cylinder two long piston rods proceed across the vessel to a cross head working in guides; and from this cross head a connecting rod returns back to the centre of the vessel and gives motion to the crank. The piston rods are placed in the piston diagonally, and one of them passes above the crank shaft, and the other below the crank shaft. The cross head lies in the same horizontal plane as the centre of the cylinder, and a lug projects upwards from the cross head to engage one piston rod, and downwards from the cross head to engage the other piston rod. The air pump is double-acting, and its piston or bucket has the same stroke as the piston of the engine. The air-pump bucket derives its motion from an arm on the cross head, and a similar arm is usually employed in engines of this class to work the feed and bilge pumps. For mercantile purposes, where the engine has not to be kept below the water line, the inverted engine is perhaps the form to be preferred.

Q. Is not inconvenience experienced. in direct-acting screw engines from the great velocity of their motion ?

A. Not if they are properly constructed; but they require to be much stronger, to be fitted with more care, and to have the bearing surfaces much larger than is necessary in engines moving slowly. The momentum of the reciprocating parts should also be balanced by a weight applied to the crank. A very convenient arrangement for obtaining surface is to form the crank of each engine of two iron or steel discs cast with heavy sides, the excess of weight upon the heavy sides being nearly equal to that of the piston and its connections. When the piston is travelling in one direction, the weights are travelling in the opposite; and the momentum of the piston and its attachments, which is arrested at each reciprocation, is just balanced by the equal and opposite momentum of the weights. One advantage of the horizontal engine is, that a single engine may be employed in small vessels, whereby greater simplicity of the machinery and greater economy of fuel will be obtained, since there will be less radiating surface in one cylinder than in two. There will also be no liability with one engine of the parts of one being broken by the other.