condenser was drawn through a copper worm, carried with many coils through a cistern of cold water, so that when it arrived at the end of this pipe it was reduced nearly to the temperature of the atmosphere. The pipe was thus brought to the vessel of distilled water already mentioned, and the water supplied by it replaced. The water admitted to the condenser through the condensing jet being purged of air, a small air-pump was sufficient, since it had only to exhaust the condenser and tubes at starting, and to remove the air which might be admitted by leakage. Mr. Howard stated that the condensation took place as rapidly and perfectly as in the best engines of the common kind.

An engine of this construction was in the spring of 1835 placed in the government steamer called the Comet. It was stated, that though the machinery was not advantageously constructed, a part of the engine being old, and not made expressly for a boiler of this kind, the vessel performed a voyage from Falmouth to Lisbon, in which the consumption of fuel did not exceed a third of her former consumption when worked by Boulton and Watt's engines, the former consumption of coals being about eight hundred pounds per hour, and the consumption of Mr. Howard's engine being less than two hundred and fifty pounds of coke per hour.

The advantages claimed for this contrivance were the following: first, the small space and weight occupied by the machinery, arising from the absence of a boiler; second, the diminished consumption of fuel; third, the reduced size of the flues; fourth, the removal of the injurious effects arising from deposit and incrustation; fifth, the absence of smoke.

(218.) The method by which the greatest quantity of practical effect can be obtained from a given quantity of fuel must, however, mainly depend on the extended application of the expansive principle. This has been the means by which an extraordinary amount of duty has been obtained from the Cornish engines. The difficulty of the application of this principle in marine engines has arisen from the objections entertained in Europe to the use of steam of high pressure under the circumstances in which the engine must be worked at sea. To apply the expansive principle, it is necessary that the moving power at the commencement of the stroke shall considerably exceed the

resistance, its force being gradually attenuated till the completion of the stroke, when it will at length become less than the resistance. This condition may, however, be attained with steam of limited pressure, if the engine be constructed with a sufficient quantity of piston-surface. This method of rendering the expansive principle available at sea, and compatible with low-pressure steam, has recently been brought into operation by Messrs. Maudslay and Field. Their improvement consists in adapting two steam-cylinders in one engine, in such a manner that the steam shall act simultaneously on both pistons, causing them to ascend and descend together. The piston-rods are both attached to the same horizontal cross-head, whereby their combined action is applied to one crank by means of a connecting rod placed between the pistons.

A section of such an engine, made by a plane passing through the two piston-rods P P' and cylinders, is represented in fig. 127. The piston-rods are attached to a cross-head c,

which ascends and descends with them. This cross-head drives upwards and downwards an axle D, to which the lower end of the connecting rod E is attached. The other end of the connecting rod drives the crank-pin F, and imparts revolution to the paddle-shaft G. A rod H conveys motion by means of a beam I to the rod K of the air-pump E. (219.) Connected with this, and in the same patent, another improvement is included, consisting of the application of a hollow wrought-iron framing carried across the vessel above the machinery, to support the whole of the bearings of the crankshaft. A plan of this, including the cylinders and paddlewheel, is represented in fig.128. The advantages proposed by these improvements are simplicity of construction, more direct action on the crank, economy of space and weight of material, combined with increased area of the piston, whereby a given evaporating power of the boiler is rendered productive, by extended application of the expansive principle, of a greater moving power than in former arrangements. Consequently, under like circumstances, greater power and economy of fuel is obtained, with the further advantage at sea, that when the engine is reduced in its speed, either by the vessel being deeply laden with coal, as is the case at the commencement of a long sea voyage, or by head winds, more steam may be given to the cylinders, and consequently more speed imparted to the vessel, all the steam produced in the boiler being usefully employed.

(220.) Another improvement, having the same objects, and analogous to the preceding, has been likewise patented by Messrs. Maudslay and Field. This consists in the adoption of a cylinder of greater diameter, having two piston-rods P P', as represented in fig.129., of considerable length, connected at the top by a cross-head c. From this cross-head is carried downwards the connecting rod D, which drives the crank-pin E, and thereby works the paddle-shaft s. In this case the paddle-shaft is extended immediately above the piston, and the double piston-rod has sufficient length to be above the paddle-shaft when the piston is at the bottom of its stroke This improvement is intended to be applied more particularly for engines for river navigation, the advantages resulting from

it being that a paddle-shaft placed at a given height from the bottom of the vessel will be enabled to receive a longer stroke of piston than by any other arrangement now in use. A more Fig. 128.

compact and firm connection of the cylinder with the crankshaft bearings is effected by it, and a cylinder of much greater

Fig. 129.

diameter may be applied by which the expansive action of steam may be more fully brought into play; and a more direct action of the steam-power on the crank with a less weight of materials and a greater economy of space may be obtained than by any of the arrangements of marine engines hitherto used.

(221.) Mr. Francis Humphrys has obtained a patent for a form of marine engine, by which some simplification of the machinery is attained, and the same power comprised within more limited dimensions. In this engine there is attached to the piston of the cylinder, instead of a piston-rod, a hollow casing D D (fig. 130.), which moves through a stuffing-box G, constructed in a manner similar to the stuffing-box of a pistonrod. In the figure, this casing is presented in section, but