CORRESPONDENTS who expect to receive answers to their letter8 must, in all cases, sign their names. We have a right to know those who seek information from us; besidetassometimes happens, we may prefer to address correspondents by mail. SPECIAL NOTE.—This column is designed for the generalinterest and instruction of our reader s,not forgratuitous 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." $WAll reference to back numbers should be by volume and page. A. S. W. A., of Mo., asks " what power would be gained on an engine with a single slide valve having twice the capacity for exhaust as for inlet, the valve having one and a half inches throw, opening the inlet port half an inch, and the exhaust one and a half inches?" We cannot see in what respect this proposed valve differs from many single slide valves in common use. As to the "gain" of power there is none whatever; all engines should haveabundant exhaust space orlargecx-haust ports. J. O. S., of N. Y.—"If large drivers were best on freight engines why are they not used ? " You know—or at least railroad men know —that for loads small drivers give the best results, while large wheels give speed, not power. The reason is apparent. E. T. of Pa.—Your drawing and description is almost identical with others which have been devised for rotary engines and for other purposes. If you write to Pratt, Whitney, and Company, Hartford, Conn., you can procure an engraving of a much superior, because simpler, device used successfully as a pump and water motor, but never considered by the inventor, Mr. Stannard, one of the firm, as suitable for a steam engine; yet it is better than your plan. One great difficulty in the production of a good rotary engine is the excessive friction, and another the excessive amount of steam required. Overcome these, the annoyances and stumb ling blocks of your predecessors, and then you may look for success. W. W. P., of Mass.—Brass, either a rod or pipe, expands in length more than iron at the same temperature. Brass expands from 82" Fah. to 212 Fah. 1 in 536, and Iron 1 in846. M. L. R., of Col., says that to prevent kerosene lamp explosions the holes in the net or screen under the chimney should be made as large as possible to admit more air. This may be done by reaming them out with a hand reamer. The amount of oxygen admitted to the flame he thinks is usually too small. E. S. N., of Mich.—In Vol. XII, page 151, we published an article on the " Pressure of a slide valve," to which we refer you as a reply to your interrogatory. As you are an" old subscriber," undoubtedly you have the volume. J. P., of Pa.—One of our correspondents writes that the best hardening pickle he ever used was springwater made into a brine strong enough to float an egg, then boiled to precipitate the lime and allowed to cool. J. C. M., of Ohio.—The following are the most amusing and easily prepared sympathetic inks: Yellows—Sulphate of copper and sal amoniac equal parts disolved in water. 2d. Onion juice. Both colorless when first applied, but visible when heated. Black—A weak infusion of galls, show upon application of a weak solution of proto-sulphate of iron. 2d. a weak solution of proto-suluhate of iron ; gives a blue when moistened with a weak solution of prussiate of potash; black, when moistened with infusion of galls. Brown or yellow—Very weak solutions of nitric acid, sulphuric acid, muriatic acid, common salt, or nitrate of potash. Shows when heated. Green—Solution of nitro-muriate of cobalt, appears when heated and disappears again on cooling. Eose-red—Acetate of cobalt solution with the addition of a small quantity of nitrate of potash, appears and disappears alternately on heating and cooling. Solutions of nitrate of silver and terchloride of gold, become permanently dark on exposure to sunlight.