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these drawbacks, it is found from practice that the non-condensing engine does nearly the whole of the work, and in many instances drags forward (if we may use the expression) the piston of the old condensing engine along with it. The work done by the larger engine is, therefore, nil, or little more than what is gained by vacuum and condensation. In the Woolf or compound principle, this is not the case to the same extent; but it yet remains to be solved what benefit there is in a more expensive and more complicated construction, when the same advantages can be obtained by the single cylinder.

That is the question for solution, and to which the advocates of the double cylinder reply, that in working with high-pressure steam, the force applied to the piston of the first cylinder is diffused over a much smaller area, and the action is less severe upon the working parts of the engine than if forced, with the relocity of impact, upon the surface of a greatly enlarged piston, as in the case of the single-cylinder condensing engine. This, to a certain extent, is correct, only it does not effect the economy but simply the strength of the working parts of the engine. Again, it is stated that the double-cylinder engine produces from the same cause a more uniform motion than the single-cylinder engine. But the advocates for the single-cylinder system affirm that these objects are all attained, first, by cutting off the steam at a point that will produce the same rate of expansion as in the compound engine, and this, although suddenly effected, is fully compensated by the action of the fly-wheel, at the greatly increased speed of the engine.

We have been the more particular in this description, as the question is not yet settled amongst practical men which of the two systems is the best ; each side has its advocates, with proofs which they adduce in confirmation of their respective theories. Without entering further into this question, we may, however, state that we would prefer the single-cylinder engine, where the advantages are the same as those of a more complicated form ; for it appears to us that no benefit is gained in the shape of economy by the double or the compound engine; on the contrary, we are inclined to believe there is a loss in the former, owing to the difficulty of working them together as one engine. The same reasoning will apply to what is called the M Naught principle, which consists in placing a high-pressure cylinder at half-stroke, under the main working beam of the ordinary condensing engine, and exhausting the steam from one cylinder to the other, on the same principle as already described in the double, horizontal, and vertical system.

Having described the different forms and conditions of our stationary engines, and the improvements that have been effected by the introduction of high-pressure steam worked expansively, we may conclude this part of the subject by observing, that we are far from arriving at that point of economy in the use of steam which an increased pressure and a still greater expansion is calculated to attain. It is true that the danger of explosion may be increased, and so it would with our present means; but in our locomotive engines we already work steam at 2001b. pressure on the square inch with greater safety than is done in our stationary engines at a reduced pressure; it is, therefore, evident that we are behind in this department, and a wide field is still open for improvement. It is not our province in this article to point out how this can be accomplished, but we may safely affirm that the improvements already attained are only the precursors of others of much greater importance in the economy and use of steam.

Marine Engines. In this department of constructive art this country stands pre-eminently forward in advance of all others at the Exhibition. It is not the principle nor yet the power that attracts notice, but the application, design, and construction of a machine calculated to propel a vessel of 6,000 tons burden, at a rate of from fourteen to fifteen knots an hour, and yet so small and so compact, compared with the magnitude of its force, as to excite the admiration of every beholder. In this department of mechanical science we have, concentrated within a space little more than twenty feet square, a force equivalent to 2,600 indicated horses' power, and that with all the conveniences of approach to every part of these powerful machines.

For examples of engines of these colossal dimensions and compact form, we have only to refer the reader to those of Penn, Maudsley, Rennie, Ilumphries, and others, to convince him of the superiority of their construction, the mathematical accuracy with which they are designed, and the precision with which they have been manufactured. In these respects they are superior to anything before accomplished in this country. Several specimens of a different kind have been exhibited from France and other parts of the Continent; but they are not the best examples of the industry of those countries, excepting some small engines from Sweden, and a beautiful double-acting engine for river boats, by Messrs. Escher & Co., of Zurich. As a whole, the marine department has been well represented by engines of great power, and working models (chiefly by Maudsley & Field) of great beauty.

If our limited space permitted, we might have gone more into detail, showing the various contrivances of the working parts; under the circumstances, we must confine ourselves to the following outlines of the different types of marine engines, as exhibited by the different makers, showing the space they occupy in the ship, length of cranks, &c., as per sketch, taken from Mr. Mallet's report in the Practical Mechanics' Journal.

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From the above it will be seen that No.1, with the double piston-rod and reversed connecting-rod, occupies the least space in length; No. 3, with a three-crank connecting-rod, the next; and subsequently follows No. 2, trunk engine, in the order of length; and, lastly, No. 4. Mr. Mallet classes them differently by taking the crank as the unit of measure; from which it follows, that the direct-acting engine, with short connecting-rod, occupies a much less total length than either of the others. This saving of space on board ship is always a desideratum in the construction of marine engines, and is a point carefully attended to in the British navy.

Most of the engines are of the screw-propeller kind, and

the specimens exhibited are certainly of the highest order, whether as regards design or workmanship. The working parts are chiefly composed of wrought iron with a link-motion for working the valves. Paddle-wheel engines are not exhibited, excepting only in models, and a pair of oscillating engines, including drawings and models of Maudsley's annular cylinder engine. Most of these have been in use for years, and are, therefore, deficient in novelty when compared with the more compact form of the screw-propeller class. Altogether the Exhibition in this department is replete with admirable specimens of marine constructions, and we have only to instance the 600 nominal horse - power engines by Penn, and the 800 horse-power by Maudsley & Field for the ironplated frigate Valiant, with others of less power, to be assured of the wonderful development of mechanical science in this age of progress.

io marine constructions we have to add a great variety of vertical, horizontal, and angular engines, adapted to almost every possible purpose where power is required. Some of these engines have double, high, and low pressure cylinders, with and without condensers; others are single cylinders of peculiar construction, and exhibit several new arrangements accompanied with surface-condensation * and other contrivances for superheating steam † and preventing the escape of heat. All these are improvements on the past; but those which have been effected during the last seventy years (although valuable in themselves) are not such as affect the general principle arrived at by Watt, and subsequently perfected by the same comprehensive intellect that made the steam-engine what it now is, the strong arm of power and the hard-working agent of civilized existence.

IVater-Power.—Half a century has scarcely elapsed since water was the prime agent as a motive force. To that element, and to wind, we had recourse when power was required for the purposes of mining, agriculture, or manufacture ; but the limitation of supply and requisite height of fall required for power were so great, and so uncertain, as to cause frequent stoppages of the works, and to spread them widely distant from cach other over the face of the country. At the commencement of the present century, when the improved machinery of Arkwright and Crompton created a demand for power on a large scale,

* Surface-condensation is a vessel where the steam is condensed by pumping cold water on to the exterior surfaces of tubes through which the steam passes.

+ Steam is superheated by passing the pipes containing the steam through flues or vessels containing air at a high temperature.

the whole of the river and mountain districts were searched for suitable sites for mills; and for a series of years it was found necessary to take the mill to the power and not the power to the mill, as it has been since the introduction of steam. In some countries it is still desirable to employ water as a moving power, and hence follow the numerous improvements that have taken place in the construction of water-wheels, turbines, and other hydraulic machines. As late as 1830 water-power was still in demand in this country; but from that time to the present it has been considered of little value unless it be in some districts where the power is considerable and where the weirs, aqueducts, and mill dams already exist. For a number of years the mills for cotton, corn, and flax were turned by water, and the construction of water-wheels was greatly improved by making them entirely of iron, which took place at the beginning of the present century. The late Mr. T. C. Hewes was one of the earliest improvers of water-wheels on the suspension principle, and this was followed by a new system of ventilation, by which the buckets were relieved of air on the entrance of the water, and subsequently restored at the lowest point of discharge. By this ingenious but simple contrivance the duty of the water-wheel was raised to a maximum, and the construction of water-wheels composed of iron arrived at a very perfect state. It was at this time that the investigations of Poncelet and the perseverance of Fourneyron introduced the horizontal wheel, founded on principles established by the former and adopted, after careful experiments, in the shape of the turbine by the latter. For several ycars the turbine made slow progress, as its advantages were not superior to that of the water-wheel, which maintained and still retains its reputation in regard to the amount of work done with a given quantity of water. The turbine, however, occupied less space, was somewhat cheaper in its original cost, and became general on the Continent and America. Of late years the turbine has been greatly improved in this country, and we have several admirable specimens in the Exhibition, a few of which it will be proper to notice.

In every description of machine recipient of water it is a universal condition that the water, to attain its full efficiency, should make its entrance and take its departure at a slow velocity. This has never been done with the turbine, but a close approximation has been attained by the best-constructed water-wheel. Smeaton, in his experiments on water-wheels, attained something above 80 per cent. of the theoretical fall, and some of the best wheels of the present time have arrived close upon that point of efficiency. Now, the best-constructed turbines seldom exceed from 65 to 70 per cent. There are,

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