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the very serious evil referred to, has an important effect in lessening the first cost of the boiler, insomuch as it enables the tubes to be securely and perfectly fixed, simply by a very slight expansion of their upper ends, without the use of any ferules or other objectionable arrangement. To fix the tubes all that is necessary is to make the holes in the tube-plate slightly conical, their diameters at the lower surface of the plate being equal to those of the tubes, and their upper diameters somewhat larger, and then, after having slightly expanded the upper end of the tube, to place it in its intended position, and by means of one or two blows with a hammer upon a steel conical mandrel inserted in its upper end, to further expand the tube, so that it may fill and jam itself in the hole in the plate.

The facility with which a tube may thus be fixed is a matter of great importance in case of injury occurring to a tube during the working of the boiler, as at the worst it involves only a trifling delay before the damaged tube can be replaced and the boiler again put in perfect working order, without its being necessary even to call in the assistance of a skilled workman, a class of individuals generally found to be unobtainable at the very time when most urgently wanted.

The boilers for stationary purposes as already described, though calculated for steady and continuous working, and economical in consumption of fuel, are not, at the same time, so heavy or cumbrous as to preclude their being used with advantage for various description of portable engines, as, in point of fact, they are lighter than those ordinarily used for such purposes, and, where extreme lightness is not of paramount importance, are certainly the most desirable form of boiler to employ. There are cases, however, and that of steam fire engines is one of them, in which extreme lightness and portability in the boiler, combined, with a power of raising steam with great rapidity, are of more importance than economy in fuel. In such cases it is obvious that the weight of the water casing, and of the water contained in it, would constitute a comparatively serious objection to the employment of the water cased boiler, which would be also further objectionable on account of the extra time required to heat the additional quantity of water contained in the casing. For purposes, therefore, of the kind referred to, the boiler used is of the type represented at Fig. 4, in which the water casing is entirely dispensed with, and the tubes used -which are of smaller diameter than those used in the other types-are so arranged as that the lower ends of the outer circles constitute an almost continuous casing around the fire-grate.

In working these boilers, a steam jet is used as soon as steam appears, for the purpose of urging the draught, and is continued in action until the engine is started, when the jet is shut off and the draught is maintained by the exhaust steam which passes by the pipe into and through the hollow baffle, whence it issues into the chimney. These boilers were designed for and are used with the greatest success in the steam fire engines manufactured by Messrs. Merryweather & Sons, whose small class engines, weighing in all about 30 cwt., raise steam from water, which, at starting, may be little above freezing, to a pres

sure of 100 lbs. on the square inch in 9 minutes, and maintain the pressure undiminished while doing absolute work, in raising water to the extent of over twenty horses power. The engine Sutherland, which weighs complete only 2 tons 18 cwt., is fitted with a boiler of the same class, and has shown wonderful rapidity in raising and generating steam. Thus, at the Crystal Palace, this engine raised steam from cold water to a pressure of 100 lbs. on the square inch in ten minutes, and subsequently threw a jet from a 13-inch nozzle vertically to a height of 200 feet, and maintained it steadily up to a height of more than 180 feet, for over five and twenty minutes, showing itself capable of continuing to do the same for any length of time desired. The boilers of the first named class of engine are 2 feet 3 inches in diameter, and, inclusive of furnace, are only 4 feet 6 inches high, while that of the latter is 3 feet 6 inches in diameter, and 4 feet 6 inches high. In order to obtain the required large amount of steam from boilers of such disproportionately small dimensions as these, it is obviously necessary that the heat of the furnace be of the most intense character, and it is equally obvious that, in order to maintain it to that intensity, a very large supply of air must be drawn through the burning fuel, and as this can only be done by means of a proportionately fierce draught, it naturally follows that a considerable per centage of heat is wasted, not, indeed, by reason of any deficiency of absorbing power in the boiler, but on account of the disproportionate strength of draught required to draw the necessary amount of air through the furnace, and which, consequently, not only compels the rapid rush of too large a proportion of the heated gases up the chimney, but also, at the same time, carries with them a by no means insignificant amount of the burning fuel itself. Yet, notwithstanding the disadvantages under which they thus necessarily labor, and which are in their nature so inimical to economy in fuel, the workings of these boilers show results quite equal to those of the majority of ordinary stationary boilers, and therefore form another convincing proof, if any were required, of the correctness of the principle upon which their construction is based.

A consideration of what takes place in the working of the Field boiler will at once clearly show why it should in all cases contrast favorably with other boilers employed under similar circumstances. Thus, taking for example the size of stationary boiler known as 80-horse, but which will in reality work with ease up to 120 horse power, the outer diameter of this boiler is 6 feet 6 inches, and its height 8 feet 8 inches. It contains 490 square feet of tube surface, the outer tubes being 2 inches in internal and 2 inches in external diameter, and the inner 1 inch in diameter. Now, upon lighting the fire, the water in these tubes immediately commences to circulate every increment of heat, however trifling, added to the water contained in the annular spaces between the inner and outer tubes, lessening its specific gravity and causing it to ascend, and cold water to consequently descend the inner tubes to supply its place. This action goes on increasing gradually in rapidity until ebullition commences, at which time the velocity of flow is increased enormously, owing to the great difference between the specific gravity of

mixed water and steam ascending in the annular spaces, and that of the solid water descending the inner tubes.

Taking the velocity of flow down the inner tubes at 10 feet per second, and the number of tubes at 289, we shall have a quantity of water equal to about 96 gallons passing down into the furnace, and being submitted to its most intense action in every second of time. Moreover, owing to the principle of action of the tubes, the water so submitted necessarily belongs to the less heated portion of the contents of the boiler, and consequently possesses the greatest capacity for heat. Now, when we consider that an amount of water equal to the entire average contents of the boiler is thus passed into the furnace, with the intervention of only one-eighth of an inch of metal between itself and the fire, in every six seconds of time, some idea may be formed of the immense rapidity with which the heat of the furnace may be passed into the water, which may be further strengthened by reflection on the well known fact, that if it be attempted to harden a tolerably large piece of steel by plunging it when hot vertically into cold water and hold it motionless in that position, the attempt will prove a failure, inasmuch as the water will fail to carry off the heat from the steel with sufficient rapidity to effect the hardening, but if, instead of holding the steel motionless as described, we move it more or less rapidly from side to side through the water, the hardening will be at once effected. Now, this is precisely the difference between the ordinary kinds of boiler and the one which forms the subject of this paper. In each case we have, on the one hand, a mass of water subjected to heat, and unable to change its position with sufficient rapidity to absorb the heat presented to it; and on the other, a constantly changing surface of fluid, carrying off the heat from the metal with great rapidity, and in case effecting the object desired.

Here, then, is one very obvious reason for the economical results achieved by these boilers, even under circumstances which might seem to put economy out of the question, the fact being simply that, in ordinary boilers, the circulation, left to shift for itself, has great difficulty in becoming of a decided character in any direction, so that the water, instead of taking off, or, as it may be expressed, rushing off with the heat from the metal as required, hangs about it, and with comparative slowness conducts it away. The consequence is that much of the heat passes into and away by the flues instead of into the water, and thus it is, as was before remarked, that slow combustion with ordinary boilers necessarily shows to better advantage than quick. It is important in discussing the merits of any boiler to advert to the difficulties, if any, likely to arise from the deposit on its surface of these troublesome matters, the sulphates and carbonates of lime, or other such compounds of them as are usually thrown down in the form of a hard scaly incrustation. It is well known that such deposits are encouraged by sluggish circulation, and that where the circulation is most feeble, as well as in the neighborhood of the feed pipe where the water first enters the boiler, the deposit is usually thicker than at any other places. Bearing this in view, we are naturally led to expect that the effect of very rapid