We have received the following-letter from the party who gained the first prize for the largest list of subscribers, and are rejoiced to find that a hard-working mechanic has been the lucky man. It will be seen from his letter that he has not lost any time while can. vassing lor us ; and we will undertake to say the time he has spent in reading the Scientific American has not been lost either. If our subscribers would only exert themselves a little in diffusing a knowledge of our paper among their acquaintance, we have no doubt that the circulation would be immensely increased. It is evident from what our correspondent says, that its advantages are immediately felt and perceived by those to whom it is made known :— ” Messrs. Munn&Co.—Your favor ef the 13th inst. was gratefully received, containing the announcement that the first prize for the largest list ofsubscribers was awarded to me ; it was somewhat unexpected, though I fondly hoped to be the lucky man. The Scientific American is truly a valuable source of knowledge—the reading of which commends it to every intelligent man, and I found that it required no extraordinary gift of gab to convince those who would give it their attention. I shall endeavor, wherever I may be located, to introduce it among my acquaintance, and to extend its patronage. As you wish me to indicate my preference either for the Pitcher or tho! money, I would state that I am a mechanic, dependent upon my daily toil for a livelihood—and you are aware that mechanics' means, at best, are small—and therefore choose the money. I may as well add that I obtained my subscribers without losing any time from my business. Permit me to thank you for your promptness and gentlemanly attention to myself although to you a stranger. John Marston. Saratoga Springs, N. Y., Dm. 16, 1852,imerxctttt. Machinery and Tools as they are.—The Steam Engine. (Continued from page 107.) Steam Boilers—To a person unaccustomed to consider the subject, it may appear, at first sight, a matter of small moment as to the form in which a steam boiler should be made, but when it is remembered that a large manufac- t uring firm will expend several thou sand dollars for' fuel in the course of a year, or that a large steamship may require a thousand tons of coal in the duration of a voyage, it is evident that a strict attention to economy in this matter is very requisite. The question, therefore , is—how can fuel be best saved a problem to be solved by attention to two circumstances, namely, the generation ot steam in the boiler and its subsequent use in the engine. The former branch of economy is dependent upon two facts,—the evolution of heat by the combustion of the fuel in the furnace, and the subsequent absorption of this heat by the water. Notwithstanding the progress of steam machinery and the vast practice which many engineers have possessed, it is certain that there is no precise theory for the steam-boiler, from which all the necessary data can be derived 1(11' the construction of a boiler of any given eva porating power, The greater part ot the rules followed by boiler makers are founded on the practice of their predecessors, and as they generally make a boiler larger than necessary, the chief requisite, a sufficient supply of steam is easily obtained, although often at an unnecessary expenditure of fuel. In discussing this subject we shall be gin with marine boilers: these have, ot late years, undergone considerable change as to form, which, in this country has been induced by the desire to use anthracite coal instead of wood for the coil sum ption 01 river and lake steamers. A similar revolution has been effected in the boilers of European vessels, caused by the desire to use stew of a higher pressure, and to diminish the space occupied by this indispensable adjunct to the engine, and also to lessen the weight. All these improvements have tended to discard the use of rectangular flues, and to introduce the employment of tubes, sometimes of small diameter, while in other in stances a few tub":; of larger diameter, and made of boiler- plate, are used instead. The boilers of our yver vessels are generally cylindrical, except the part for the furnaces, two rows of large boiler-plate tubes convey the heat through the water to the further end of the boiler, thence it returns through one or two rows of larger tubes to the front of the boiler, and so up the chimney. In other cases two boilers are placed together, fore-and-aft, and one chimney being made to serve for them both, the heat is re-conducted from the front by an additional row of tubes. With respect to the nnmher of the latter, we will rem ark that the main or lower ones are generally five in number for each furnace, being arranged to suit the cylindrical shape ot the boiler, the upper or ret urn tubes are generally placed five ill a row. There is another variety ofthe tubular boil- er,m ore especially adapted for sea-vessels because, although by no means so strong as the one we have just alluded to, it is better adapted fol"iplacing below deck, as it approaches to a rectangular shape, the top part, however, being generally more or less arched, in order to gain additional strength. In boilers of this description it is usual to employ a considerable number of small tubes, which are from two to three inches in diameter. These tubes are sometimes horizontal smd frequently vertical—many of our largest steamers employing boilers of the latter kind, some of which have given extremely satisfactory results as respects the weight of water evaporated by one pound of coal. This, in boilers of the ordinary construction varies from 8 to 10 lbs., but in some of these latter it amounts to 12'9 lbs., which may be attributed to ' slow combustion combined with the mode of arranging the flues and tubes. In those boilers which have horizontal tubes, and most are of this sort, the heated air can either pass directly through them to the chimney, or if one large passage be used for the main flue, the tubes are so disposed as to return the hot air to the front ofthe boiler, whence it passes to the chimney. Theiollowing particulars oh boil er with horizontal tubes, and intended for a low-pressure engine, will give an idea of the present state of this branch of the Steam Engine. The cylinders being 611 inches diameter, and 4 feet 6 ir.ches length of stroke, the weight of the boilers exclusive of water, was 45 tons, the tubes, which consisted of iron were 3 inches diameter and 6 feet long ; the grate-bar surface was 85 square inches per horse-power, the total tube and flue surface 16 square feet per horse-power. Of the rectangular flue boiler, formerly placed in stewers, we will say nothing, as the use of it is almost entirely abandoned. The wagon boiler for stationary engines has, in a similar manner, sunk in estimation, being supplanted by those of a cylindrical shape, but whalever the form may be, attention to certain principles is requisi te to insure a satislac- tory result. Locomotive engineers have almost totally expunged the term horse-power from their nomenclature, but as it is still retained in other branches of steam machinery, it is necessary for the boiler-maker to know its import as respects his business ; this is generally defined to be one cubic foot of water evaporated per hour, although, in fact, the power developed by this quantity is much greater in all modern engines. The necessity of “ hard firing “ will be avoided by the experienced maker, who, a ware of the consequent injury resulting from this practice will take care to allow sufficient fire surface. A cylindrical boiler, with an internal flue, is very frequently used, and in the Cornish variety the firegrate is formed in the entrance of the latter. Plain cylindrical boilers, without any inside flue and enclosed by brick-work, in which the furnace and flues are built, are extensively manufactured. In all these sorts of boilers an excessive length is uselfess, but the quality of the fuel materially influences the ratio which should exist between the diameter and the length. A cylindrical boiler with double in ternal flues and furnaces has met with considerable success. - To enumerate all the different varieties of boilers is not our intention,—before closing, however, we will mention that species which has, perhaps, exercised more ingenuity than any other—namely, that in which the water is enclosed in • tubes and surr.ounded by th” f ame and hot air; doubtless no boiler is better adapted for generating high-pressure steam with ilipidity, but its glaring deficiency, as res safety and economy, are well known. Respecting the locomotive boiler there is little to be sa id, as it has remained nearly similar in form for many years. The only modification of note is in its being often made of an ell iptical shape instead of cylindrical, thus affording a more powerful boiler without detracting from its safety.