indispensable in obtaining those ends which we have found it should fulfil.

Having regard to our limited time, I will bring before you only three complete diagrams of locomotives, and I will select for these the three most ordinary types of engines in use at the present day, leaving for separate diagrams such deviations from these types as I may be able to bring under your notice.

The first of these diagrams (fig. 8) shows a longitudinal section of a goods engine with all its six wheels coupled and acting as driving wheels, and with inside cylinders. The second diagram (fig. 9) shows an outside view of a passenger engine, having a pair of leading wheels, and having the four hinder wheels coupled to drive; the cylinders being outside.

Both these engines are intended to be used with tenders.

The third diagram (fig. 10) shows a longitudinal section of a tank engine, i.e. an engine carrying its own water and fuel.

In each case we will commence our consideration with that soul of every steam-engine-the boiler. It will suffice to employ the same description for all three boilers. The problem, it will be remembered, is to boil off when working at the very maximum some 1,600 to 2,000 gallons of water per hour, or from 26 to 33 gallons per minute. To do this it is necessary, as indeed hardly need be said, first to ensure the combustion of an adequate amount of fuel, and then to provide a sufficient surface licked,' as the French say, by the flame on the one side and wetted on the other, to carry off from the products of combustion the heat evolved, and to transmit it to the water. To consume the weight of fuel I have mentioned, there would be provided in an ordinary land engine, or even in a

marine engine, a grate area of about 100 feet. To obtain such an area with the width possible in the fire-box of a 4' 8" gauge engine (that is, possible, after making the requisite reductions for water spaces, metal, and clearance), a length of grate bar of 27 feet would be required.

Obviously, it would be impossible to satisfy such a requirement, and therefore for the combustion of the necessary weights of fuel reliance must be placed, not upon the mere area of the grate, but upon the mass of fuel contained in the fire-box. But the massing of fuel renders perfect combustion a still more difficult operation than it is under ordinary circumstances; and ordinarily it is difficult enough, as, even with a large grate area and with the fuel comparatively regularly spread, the ensuring a uniform and adequate supply of air to the fire is no easy matter, even with the use of lofty chimneys.

In the case of the locomotive, however, where, as we have said, there cannot be a proper grate area, and the fuel must be massed, the engineer is at special disadvantage, because, so far from having a loftier chimney than that which can be obtained, for a stationary engine, or for a marine engine, he is compelled to restrict himself to a funnel the very top of which must not exceed, say, 13 feet above the level of the rails. Under these circumstances he is driven to have some kind of forced draught. That draught might have been obtained, and, as we know, attempts were made to obtain it, by means of a blowing fan, but all such attempts have for some time, at all events, been relinquished, and the requisite power of draught is now universally got by means of the waste steam blast.

On looking at the section of the smoke-box, it will be seen that the orifice of the blast-pipe (the pipe which delivers

the exhaust steam from the two cylinders) is at a slight distance above the topmost row of tubes. The waste steam issuing from this orifice with the speed due to its pressure induces a current in the surrounding air, and carrying that air with itself up the funnel at a high velocity, there is necessarily obtained a partially vacuous condition in the smoke-box.

William Nicholson in 1799 translated Venturi's work upon the lateral communication of motion, in fluids. In this work, induced currents are mentioned as being used for the draining of marshes, while the application of the lateral or induced current in water for the purposes of moving larger bodies of water was referred to in Mr. Unwin's Lectures, delivered here in 1869.

In 1806 Nicholson patented the exciting the draught of a fire by means of an induced current caused by a jet of steam. I have here a rough model, made out of an ordinary blowing fan, to exhibit the production of an induced current.

To revert to the blast in the locomotive, it is by means of this, involving no machinery or moving parts of any kind, that the requisite power is obtained for forcing the air through the fire; but it is not found desirableespecially with coal-to introduce the whole of the air to the fuel from below; as if this were done, two objections would arise, one, that the air in its passage through the fuel, after having produced carbonic acid by perfect combustion in the lower part, might in the upper part take up a second portion of carbon and become converted into carbonic oxide, and then pass away with the carbon contained in it unconsumed; the other, that in burning coal, when the coal is freshly introduced, the hydro-carbons distilled off might also pass away without being consumed, causing an offensive smoke and some loss

of fuel. It is therefore in practice found to be desirable to admit a very considerable portion of air above the fuel. The regulation of these two inlets of air is made, for the one, by means of dampers applied to the ashpan, and for the other, by means of slides or other analogous contrivances applied to the fire-door opening.

While on this subject, it may be well to allude to the smoke difficulty. Unhappily for the comfort of railway travellers, it was discovered some twenty years ago, that engines could be worked as effectually and in most districts more cheaply, by substituting raw coal for the coke previously employed, and, except under certain special circumstances or in particular districts, raw coal is the locomotive fuel of the present day. Efforts are made to burn the smoke, and when the firing is carefully done and the engine is not too much over-taxed, these efforts may meet with a very considerable share of success in obtaining the desired end even with a bituminous coal; but when I travel by railway I must say I am glad on looking at the tender to find that the coal being used is of an anthracitic character, and to see there the smokeless Welsh coals, the highest class employed in steam navigation.

Various plans have been resorted to for smoke combustion, but they may practically be said to have settled down into the use of the 'Brick arch' and of the 'Deflector' shown at 'A' 'B' on the diagram (fig. 8). The brick arch 'A' extends backwards inside the fire-box from the tube plate for about two-thirds of the length of the fire-box, while it spans the whole width of that box. The deflector 'B,' an-shaped wrought iron pipe, is either fixed in the fire-box as shown, or, more commonly, is a mere loose inverted scoop inserted into the fire-door opening. deflector points towards the hinder end of the brick arch; and the combustible gases, be they carbonic oxide or the

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