to introduce a remedy. The condenser at the side of, or below, the pump is in one of the worst positions that can be conceived; the idea of allowing the condensed steam to fall, only to be raised again, seems, on consideration, to be foreign to the ideas of our talented engineers. It is well known that, in ordinary arrangements, the condenser is always in the position alluded to; steam even of a low pressure is larger in volume, but not as dense and heavy as water; it is also more elastic, hence it will more readily ascend. This, then, being clearly understood, it is not unwise or impracticable to assume, that if the condenser were on the top of the air-pump, instead of at its side or bottom, a better vacuum would be maintained. I beg to offer a description of an arrangement of condenser and position of the valves, both for correct action and accessibility. It will be understood that the condenser in this case is over the air-pump; the suction-valves are inverted, consequently the weight of the water assists the action of the piston in causing a vacuum. The exhaust-steam from the cylinders rushes up the exhaust-pipe, and enters on the top of the condenser. The water in the air-pump is discharged through the delivery-valve, at the top of the pump, and from thence through the delivery-valve at the ship's side. A door is secured opposite the deliveryvalve, and doors are provided on each side below the bottom of the condenser, for the double purpose of inspecting the suction-valves and the air-pump piston. This arrangement of condenser and air-pump will occupy as little room as those of the ordinary kind, with the advantage of accessibility to all the working parts without disarrangement. It may be argued that the stuffing-box, being in a recess when used for guides, would be troublesome to keep tight or repack, but if oil be always kept in the recess, so as to entirely cover the gland, In the days of the introduction of side lever engines, it would tend to lessen the liability of leakage; the nuts the arrangement of the condenser and air-pump was faulty; of the gland and bolts could be adjusted by a box-spanner, in some cases the foot-valves were almost inaccessible. or the bolts prolonged to the top of the condenser. In Not many years ago, being on board a steam-ship fitted cases where the depth of the ship would admit, the recess with old side lever engines, which were then undergoing could be dispensed with; trunks are not proposed for this repair, I noticed a rope and block tackle over near the arrangement, as their diameters would be necessarily incondenser. On inquiring of the engineer how he pro-creased, owing to the length required to pass through the gressed, the answer was, "I am just going to sling one of condenser, unless a recess were resorted to as now prothe men with this tackle, by the heels, to inspect the foot-posed. valves; and that," said he, "is no foolish job." On further examining the engines, I found that an upside-down attitude was required, and indeed the only one allowed for the inspection of the valves in question. Happily now, however, such an inconvenient arrangement is of rare occurrence. We also find the side lever engine is being superseded by that of the oscillating type.

frame like the letter A laid on its side, also the hollow | ordinary arrangement, it will not be deemed out of place frame, with a raised projection for the crank-shaft, and a stay from the upper portion connected to the cylinder; this last may be said to be the most simple, and, at the same time, of less material than the A frame. As before stated, the strains on the frames are continuous, yet, when sudden shocks occur, from the racing of the engines or priming of the boilers, the tenacity of the cast-iron is severely tested. As this is the case, wrought-iron might be used with great advantage, both as to increase of strength and decrease of weight. The crank-shafts of marine engines are generally of wrought-iron, in one mass, the cranks being double, and forged with the shafts. Three bearings are deemed imperative, so as to equally distribute the strains. Now, this is correct in theory and practice, and the writer will be deemed committing a grave error no doubt, in mathematics, when he assumes that the forward frame and half-crank can be dispensed with, in order to reduce the weight and material. He is, of course, aware that the thrust and pull of the connecting-rod will be thrown on the centre crank and bearing, but, in order to counteract this, the length and diameter of the shaft at that part should be increased. He would also prefer, in this case, to extend the frame and connect the upper portion to the condenser; the cap being on the top instead of at the end, as now used. Screws might be employed to adjust the side brasses; the eccentrics could be within the cranks, or between them and the bearings.

Having alluded to the principal working details, I will now lay before you a description of the mode of condensation-past and present. It is well-known that the principle of condensation is to convert the steam into its original state. The contact of the cool fluid, in the shape of water, accomplishes this in the ordinary condenser, and cooling surfaces in the surface condenser.

The next portion of the subject now before us is the ordinary condenser for screw engines. The action of the air-pump in this case is usually horizontal, consequently the valves are at right angles to the pump. To describe each arrangement of condenser and air pump that have come under the writer's notice would occupy too much time, consequently a brief mention of two or three The arrangement of the ordinary condenser and air- examples on this occasion will be deemed sufficient. For pump for oscillating paddle-engines is generally as follows: direct-acting and trunk engines, with the cylinders secured the condenser is situated between and below the trunnions together or side by side, the condensers were between, of the two cylinders; the air-pumps are at an angle, with and, in some instances, in front or at the sides of the airtrunks and connecting rods of the ordinary kind; the foot-pump. The foot and discharge valves were directly over valves are at the bottom of the barrel of the pump, the each other, the former under the pump at each end, the piston has valves in it; and the discharge-valve, when not condensed water or steam being drawn through the foot at the top of the pump-barrel, is at its side. Now, the valves and forced through those above. In another principal defects in this arrangement are in the position of instance the foot and delivery valves were extended the the valves and condenser. When the foot-valves are entire length of the air-pump and passages, the position directly underneath the pump's piston, it is obvious that an of the valves over and under being as before, and the conalmost entire disconnection must be made to inspect them. denser being between the air-pumps. For return conAlso, in the case of the piston-valves requiring inspection, necting rod engines, the condenser and air-pumps are the pump-cover must be removed, and to attain this the subject to great disadvantages. In order to obtain a gland packing has to be slackened, and the connecting rod passably good arrangement, and, at the same time, occupy disengaged. Now, to avoid these evils, doors might be a moderate space in proportion to those last mentioned, introduced, but with these disadvantages-increased the condenser, &c., have to be shaped to suit the purpose reheight or length of the air-pump passages, and a body of quired. It must be perfectly understood that when the water always above and below the piston, which un- piston rods are beyond the crank shaft (as in the doubtedly is what any right-thinking engineer would dis- examples now in question) there is a certain amount of approve of, it being clearly understood that an air-pump space required for the piston-rods and guides of the cross will produce a better vacuum when the piston thoroughly head, or guide block, whichever may be used. It is also discharges the contents between the foot and delivery-clear that accessibility to all the valves without disvalves at each stroke. arrangement should be attained. To illustrate these Having thus pointed out the existing evils of the desiderata the following examples will be sufficient for

the present purpose:-In one instance the condenser is partially between the cylinders, and extending beyond the crank shaft; the air-pumps are at the side of the condenser; the suction-valves extend the length of the airpump; and the discharge valves are between each pump, the pump and the valves being beyond the crank shaft. The next example is as follows:-The condenser and its appendages are entirely beyond the crank-shaft. The air-pumps are at the extremity or sides and near the bottom of the condenser. The foot and discharge valves extend the entire length, and are arranged over and under the pumps in the usual form. The guides for the piston rods are between the upper portion of the condenser and that of the discharge chamber.

dnced the tubular arrangement with great advantage. Engineers at that time were slow in appreciating the then presumed gain, and it is only lately that we have seen the surface condenser universally adopted by the powers that be. To condense steam properly is undoubtedly to reduce it to its natural or original state. Now, in the ordinary condenser we bring water into actual contact with the steam to condense it. Surface condensers are to be recommended, particularly for one reason, viz., the production of distilled water for the feed of the boilers. The arrangement of the tubes in surface condensers entails practical difficulties as to the position most suitable, whether they be inserted transversely, perpendicularly, or longitudinally, of the hull of the ship, renewal of the tubes being often required (sometimes while at sea) from corrosion.

The means adopted to render the connection of the tubes in the plate air-tight are numerous. The usual mode now is-india-rubber rings recessed in the plates encircling each tube compression being obtained by a nut for perpendicular tubes, and by the vacuum in the condenser for those of horizontal positions;

economical. It must here be remarked, however, that compression of the india-rubber by vacuum can only be attained when the steam is condensed by the external surface of the tubes or within the plates. The circulation of the water is either through or surrounding the tubes, and is produced by pumps with plunger-piston or centrifugal action. The position of the piston-pumps is horizontal, motion being derived either from the steampiston or piston-rod. The centrifugal-pump requires a separate engine, or spur-gearing, &c., from the crank shaft to give the required velocity.

Having disposed of the principal arrangements of airpumps and condensers as formerly constructed, allusion will now be made to those of recent improvement and practice. As before stated, a better vacuum can be at tained when the condenser is over the air-pump instead of at the side. For direct-acting engines there are two arrangements specially worthy of notice. 1st. The airpumps are worked by the steam-pistons between the cranks as near the base lines of the engine as the periphery-this is simple and efficacious, and at the same time of the circles will admit, the condenser being one chamber, directly over the air pumps. The suction valves are inverted in the bottom of the condenser, so as to effectually drain the same. The discharge chamber extends the entire length on each side and back end of the condenser, the valves being nearly in the same line as those for the suction, but reverse in action. The next example is the same as the last in principle, although different in arrangement. The air-pumps are situated as in the last example, but the condensers are separate, one to each engine, over and on each side of the air-pump. The suction valves are inverted in the bottom of each condenser to obtain the advantage before alluded to, the discharge chamber and valves being central or between each condenser, and directly over and between the airpumps. It may now be argued, that if the two examples last mentioned are perfect in action and arrangement, what is the cause of the diversity? The answer to this is, Diversity of idea. Engineers, as a rule, are averse to the act of copying from each other. No sentence grates more harshly on the ear of a scientific man than the words, "Where did you copy this," or is more repugnant to his dignity.

The values of the two arrangements now used for the condensation of the steam are about equal. In the case of the water surrounding the tubes, the steam passes through the same, and in the case of the steam surrounding the tubes the position of the water is reversed.

It is obvious that where internal condensation is effected a greater number of tubes are required, in relation to those of the external system-the inner surface of the tube being less than that of the outer. The advantage gained by the steam entering the tubes may be said to beaccess for cleaning without disarrangement. Injection, or ordinary condensers, are more generally used than those of the surface kind, on account of economy in the outlay of capital at the commencement.

The injection-valves for ordinary condensers are generally of the solid or gridiron type, the latter to reduce the stroke to open and close. The pipe for the dispersion of the water is usually a tube, with apertures, of an elongated or circular form. An improvement has lately been made in these pipes. by contracting the area for one-half the length, thus equalizing the diffusion of the water throughout.

Having referred to the improvements in the arrangement of condensers, &c., for direct-acting engines, attention will now be given to those adopted for double Piston-rod engines. It must be borne in mind that for this class of engine the prolongation of the piston rods beyond the crank-shaft greatly deteriorates the arrangement of the air-pumps and condensers, in relation to the space occupied by those for single piston rod engines. In the examples now given, the air-pumps are worked by the steam piston, and as near the base line as possible. The The next valve necessary for the condenser is the sniftcondensers are separately arranged outside the guides of ing valve, which is a single disc of gun metal, with a the piston rods of each engine; the suction-valves are in- slight spiral spring at its back, or upper part. A screwed verted above the top of the air-pump, as in the last ex-spindle is universally used to prevent the valve from rising, amples; the discharge chamber is between the air-pumps and the valves, on the same level as those for the suction. It will thus be understood that both suction and delivery are at the side, over and extending the length of each airpump, instead of being directly over them, as in some

cases.

The next example worthy of notice is arranged as follows:-The condensers and their appendages are beyond or outside the guides of each engine, the air-pumps deriving their motion as in the previous examples, and are as near the base line as possible, so situated as to clear the guides. Partially over and beyond the side of each air pump are the discharge valves, above which is the discharge chamber; over this, and at the side of the same, is the condenser with the suction valves inverted.

I will now allude to the system of condensation known as surface condensation. Mr. Hall, in days of yore, intro

after the water and air in the condenser has been blown out previously to starting the engines. It might be deened neglectful if I were not to make allusion to the bilge injection-valve or cock, whichever may be used. This valve, as well-known, is only required in cases of necessity, such as leakages, or disarrangement of the bilge or donkey pumps. I would beg to suggest that the bilge water should not be allowed to enter the condenser, on account of the generally impure state of the bilges. A valve and box might be arranged at the end of the airpump for this purpose.

The portions of the marine engine next for exemplification are the feed and bilge pumps. The position of these is so arranged that a free access can be obtained to the valves and surrounding parts without disarrangement. Some makers prefer to work the feed and bilge pumps in a line with each other, with one rod and plunger direct

TWIN SCREW PROPULSION.-TABULAR STATEMENT OF SHIPS, MARINE ENGINES, &c., CONSTRUCTED BY MESSRS. DUDGEON, BLACKWALL, SINCE 1851 TO THE PRESENT DATE, SUPPLIED BY THE FIRM.

TABULAR STATEMENT OF MARINE ENGINES CONSTRUCTED BY MESSRS. JOHN PENN AND SON, GREENWICH, SUPPLIED

BY THE FIRM.
Screw Engines.

The following is a list of engines constructed by Messrs. Maudslay, Sons, and Field for H.M. Navy since 1851 to the present date, kindly furnished by Joshua Field, Esq. :—

Horse-power Nominal.
Total 37,570

.....

THE FOLLOWING TABLE GIVES PARTICULARS OF SOME OF THE PRINCIPAL MARINE ENGINES, RECENTLY CONSTRUCTED BY MESSRS. R. NAPIER AND SONS, GLASGOW.

Total number constructed by this firm, from 1851 to 1864 inclusive-Screw engines, 87; Paddle engines, 39.