CORRESPONDENTS who expect to receive answers to their letters must, in all cases, sign their names. We have a right to know those who seek information from us; beside, as sometimes happens, we may prefer to address correspondents by mail. SPECIAL NO TE. This column is designed for the general interest and in struction of our readers, not for gratuitous replies to questions of a purely business or personal nature. We will publish such inquiries, however, when paid for as advertisemets at $1*00 a line, under the head of "Business and Personal" $W All reference to back numbers should be by volume and page. M. N. R, of--------It is difficult to designate a book teaching a machinist the " science, reasons, and demonstration of his business.' Such information is scattered in a number of publications. Hand-books and manuals, together with works on natural philosophy, and Byrne's Dictionary of Engineering, or similar later publications, will be useful as aids. J. B. C, Jr., of N. Y., asks for the rules for setting Stephens' cutoff. We never knew this cut-off to be set by measurement; it is done usually by trial, setting the engine on its centers, covering the ports,etc. No instruction (verbal) could give you the knowledge desired; the work belongs to an experienced engineer. J. W. H., of Iowa asks " what is the expansive force of steam when cut off at half stroke, the pressure being eighty lbs per square inch ? " The average pressure is 67*7 lbs., less 10 per cent allowed for at-t enuation of steam. J. B. S., of Ind. We cannot give the actual power of a turbine wheel unless we know its style, size, and the amount etc. of water used. Some builders claim a yield of 90 per cent of the power applied For reply to your other question we refer you to answer to " H Co., of W. Va." on page 204, current volume. J. E. B., of Ind. Portable engines as usually built the best class are as light as they can be, unless the boiler be made of sheet steel and all the connections, shaft, etc., also of steel. It is a fact that the larger the engine the less its proportional weight. We know nothing about the engine you refer to weighing only 16 lbs. topfte h-opso power. T. S. B., of Mo. The oil of tobacco may be removed in a great measure from an old meerschaum pipe by boiling it in melted tallow and wax, say about equal parts of each. T. B., of 111. The less the specific gravity of coal oil is, the more inflamable it is, but we are not aware that any exact relation between the specific gravities of such oils and the temperature at which they will ignite has been established. H. C of Toronto, Ca. The amount of gas obtained from a tun of coals, varies very mucli with the kind of coal used, and the way in which the distillation is performed. It varies from 6,500 cubic feet to 15,000. Boghead cannel is according to Hughes the richest in illuminating gases. To give the average of all the varieties would involve consider -able computation. An allowance of 25 per cent is made by some authorities for losses by leakage, condensation, etc., but we believe that in well managed works this is too large. N. O. H., of Minn. According to De Saussere, freshly burned boxwood charcoal absorbs ammonia 90 parts of its own bulk ; hydrochloric acid 85 parts; Sulphurous acid 65 parts ; Sulphureted hydrogen 55 parts nitrous oxide 40 parts ; carbonic acid 35 parts; olefiant gas 35 parts. Car bonic oxide9-4 parts oxygen 9'2 parts; nitrogen 7*5 parts ; marsh gas 5 parts; hydrogen 1*7 parts. Soluble glass is made by melting together in a Hessian crucible, 8 parts of carbonate of soda, or 10 of carbonate of potash, with 15 of pure quartz sand, and 1 of charcoal. The materials should be perfectly fused, and remain so for some time. They should be poured out before cooling into an iron vessel as otherwise it may be difficult to remove it from the crucible. It dissolves in from 5 to 6 times its weight of boiling water. It is a cheap material for lining cisterns, and is said to serve the purpose very well. W. M., of Conn., asks if we know of any steam engine without " dead points " (single engine referred to), and if constructed, simple in its parts and certain in working, it would be valuable. We have never seen such an engine. We have seen some that claimed to be without dead points (i.e. points where no power was delivered), but never saw either a reciprotating or rotary engine of that character. If you can build such an engine, " simple " and " certain " etc., trot it out. It will pay as a curiosity, if it is otherwise valueless. W. S. T., of 111., superintendent of works employing steam power, says he has tried every advertised means, or substance, to prevent incrustations on his boilers (the water being limy), without avail, un til he used white oak bark, or rather poles of that wood, and since that has had no trouble He advises others using water impregnated with lime to do likewise. "We cannot see the connection. The oak bark contains tannin and quercitric acid, neither of which we understand affects lime, unless this acid may combine with the lime to make a soluble salt. Certainly however, the oak saplings will not injure the boiler, and the remedy is simple and inexpensive enough to warrant a trial. F. W. K., of 111. There are instruments made for the calcu lation of power transmitted by belts. One is the dynamometer of James Emerson, Lowell, Mass., illustrated and described on the first page of No. 1, current Vol. Scientific Amekican, and another Neer's dynamometer, also described aud illustrated on page 296, of Vol. XVIII, same paper, address being Geo. C. Koundey, 254 Broadway, New York city. The steam engine indicator is another method of determining the power transmitted by belts. W. Y., of Mo. Cooley's recipes are considered usually reliable. Your failure is probably due to impure materials. The best cement we know for glassware is that sold as the " Diamond Cement," imported from England. J. J. W., of New Brunswick, says he derives great benefit from surface blow-off pipes for his boilers, which use salt water. He first tried them over the furnace, the hottest part of the boiler, but had much better success when he removed them to a cooler part of the boiler, having noticed the scum on the surface of water in a boiling pot to flow away from the point of ebullition. His pipes are plugged at the ends and pierced with small holes, the inner ends of the pipes being the highest. The plan is a good one, but if the pipe is jointed just inside the shell of the boiler and provided with a float to keep it always at the surface of the water the result is still more satisfactory. The blowing ofl should be attended to at least once a day. C. C. L.,of Ohio, sends us a sketch of a portion of the common hot water boiler usually placed in dwellings in connection with ranges and furnaces. He does not understand the necessity of the supplementary pipe connecting the hot water draw-off pipe above the boiler and the cold water delivery pipe at the bottom. It is evident that when the boiler is full, the cold water supply pipe which descends through the top of the boiler to within a few inches of the bottom will supply no more water until some of that in the boiler is drawn off, and, of course, circulation inside the boiler ceases. To keep up this circulation under these circumstances it is necessary to connect the hot water pipe with the Bcold water pipe that passes the water through the fire or heater. Of course, this cir-lationis, in a degree, an element of safety. Every boiler, however, should be provided with a safety valve, loaded simply to the pressure Tnecessary to raise the water the required hight within the limits of the boiler's resisting strength. A. E. W., of 111. We are sorry to say that there are not, to our knowledge, any published data on the resilience of springs. There is no reason why experiments could not be made and the results arranged in tabular form. It would be not only of very great general value, but would bring money and fame to the experimenter. C. A. L. of Tenn. Rock or swamp maple is a better step for a turbine than either lignumvitae or elm. It will sustain the weight of any turbine built. Cast iron is worthless for the purpose. Don't try it. I. N. S., Jr., of N. Y. We have once or twice given the process of blueing gun barrels and also pieces of steel. It is simply heating the piece to be blued in powdered charcoal over a fire until the requisite color is obtained. It will not injure the gun barrel if carefully performed. P. M., of Pa. A lath or shingle saw, properly fitted up, may travel at a speed that gives a velocity to its edg-e of 11,000 or even 12,000 ft. per minute. See answer to " H. Co., of W. Va.," on page 204 current volume for calculation of number of revolutions required. The rule is ; di. vide the circumference in feet by the speed desired 9,000,10,000, or even 12,000 feet and the quotion is the number of revolutions. J. P. J., of Mass. Alabaster is a delicate translucent form of .gypsum and easily contracts dirt and becomes soiled unless kept carefully under glass. When soiled it should be immersed in clear water four or five days, then in water containing a small amount of lime for about the same time, then rinsed in clear water and dried in the air. Wooden vessels should not be used as they may stain the work. If the work is jointed and the joints separate they may be re-united with plaster of Paris.