and Arizona were about 13 knots, 15 knots, and 161 knots. In connection with the subject of the size of our future sea-going steam-vessels, Mr. William Denny says (Watt Lecture, 1882): "Steamers were increasing in size, and the least costly increase for weight-carrying, and, up to certain points, for speed, was in beam, provided sufficient draughts could be obtained. Steamers would follow their natural course of development, and it would be for dock proprietors, river trustees, and harbour boards to see that their docks, rivers, and harbours were of such depth as to permit them to favour steamers so developed. He believed it was found daily more difficult to build the larger types of Atlantic steamers rigid enough for the service, even with the great percentage of their displacement devoted to structural weight. A reaction would set in against their extreme proportions and absolute length. When that happened, beam would be increased; as a consequence draught increased, and distinct preferences accorded to ports having great draught of water. Besides, great draught of water, and comparative shortness of a steamer, were more favourable to the efficiency of the screw, by keeping it well immersed, than great length with shallow draught, which told very much against the screw's efficiency." Mr. Denny further states that he he is "convinced that the steamer which was to do the Atlantic work would be a vessel of what might be called at the present time moderate length. That was a vessel which would not only be shorter than the City of Rome, but shorter than the Servia, and shorter than the Alaska, which, of the three steamers, as far as he could learn, came nearest the type he had in view. He believed the steamer to do this work would be under 500 feet in length between perpendiculars. What her other dimensions should be would have to be fixed by experiment and a very careful series of calculations." The annexed diagram of the comparative sizes of some leading steam vessels was originally used to illustrate a lecture delivered by the author during the Naval Exhibition, held in Glasgow in 1880-81, under the auspices of the city authorities. The compound engine, as introduced on the Clyde by Randolph, Elder & Co., in the first mercantile steamer fitted with this form of engine, the Brandon, in 1854, reduced the consumpt of fuel to a large extent. Later on, due to further improvements, such as surface condensation, the use of three cylinders instead of two, whereby the efforts on the shaft were more completely equalized, the consumpt was further reduced, until, in the latest compound engines, less than 2 lbs. of coal were required. Recently the triple-expansion engine has been introduced, whereby a further saving in coal is obtained. In this form of engine the expansive action of the steam is carried out through three cylinders consecutively-that is, it is admitted from the boiler into the first and smallest cylinder, and, after driving the piston of that cylinder before it for a part of the stroke, the admission of the steam is cut off, the rest of the stroke being accomplished by the expansion of the steam already admitted. Thereafter this steam passes into the next or intermediate cylinder, and presses forward the piston in that cylinder, and finally, as its expansive power is not yet exhausted, it is allowed to enter the third cylinder, pressing forward in turn the piston there. The intensity of the pressure, as it enters the second or intermediate cylinder, is much less than when entering the first; but as this cylinder is made proportionately wider, and has therefore a greater area of piston, the total driving power on the piston is still obtained about equal to that in the first cylinder. The third cylinder is in turn larger than the second cylinder. Certain proportions have been adopted as best suited to equalize the pressures on the pistons, and from thence to the cranks. The quadruple-expansion engine, of which a few have been made, is simply, like the triple-expansion engine, a development of the compound-that is to say, in the compound the steam was expanded through two cylinders, in the triple through three, and in the quadruple through four, higher pressures being required in the triple and quadruple forms than in the compound, as the range of expansion is greater. Here, however, we touch the main item in the increased economy of the later forms, and which may be shortly referred to. Generally speaking, steam when expanding may be represented as following what is known as Boyle's law of gaseous expansion-that is, that the pressure varies inversely as the volume; so that if, in the triple-expansion engine above referred to, we suppose that the steam is cut off from the boiler after driving the piston through half of the stroke, or when one-half of the cylinder is full, then at the end of the stroke the volume of steam will be double of what it was when cut off. The pressure, therefore, will be only one-half of what it was before being cut off-that is to say, the volume having doubled, the pressure has fallen to one-half. In like manner, when the volume has further increased-say to five times-then the pressure will be reduced to one-fifth. It will now be found that the mean pressure is higher when the expansion is considerable, in proportion to the steam used. Thus, if the steam were not used expansively at all the pressure would be uniform throughout the stroke; but a whole cylinder full of steam would be used. If cut off at say half-stroke, the mean pressure would now be approximately seven-eighths of what it was in the first case. But only one-half of the steam is now used. |