CORRESPONDENTS who expect to receive answers to their letters must, m 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 JVOTE.—This column is designed for the general interest and instruction of our reader s,not for arattUtous replies to questions of a purely business or personal nature. W will publish such inquiries, however-when paid for as advertisemets at $1*00 a line, under the head of "Business and Personal." B3?"Ml reference to back number sshouldbe by volume and page. L. L. Gf., of lich., and O. R., of N. Y.—Your criticism of rule for determining the pressure in a boiler whenitblowsoff, published, in " Answers to Correspondents " on page 300, current volume, arises from your misunderstanding of our use of the terms " long and short arm "of a lever. "We used these terms in preference to" leverage of the forces," because we supposed they would be better understood, premising that when employing them in a formulait seen by allthat measurement from the fulcrum was meant; so that in a lever of the third order like that of the ordinary safety valve, the long arm would be the entire length of the leVer,and the short arm the distance from the valve stem to the fulcrum. F. W., of Oregon.—The ".sett " of a wagon wheel should be such that the spokes should stand perpendicular on a level surface as they stand under the ixletree. '* Gather" we believe to be a fallacy. If however, any of our correspondents differ from us in opinion, we are open to conviction. J. F. B., of Ind.—The best way to set a horizontal boiler, is, to have the firebox at least as wide as the boiler, and have as much heating surface as possible, but below the water line. All passages should be made large; so as to allow a free passage to the heated gases, and where they leave the boiler the passage should be made so as to open or close by a damper. The bridge wall,should be highenoughto prevent the coal from be ing thrown over, and the grates should be low enough to allowampleroom for combustion. Nothing can be gained by putting the fire near the boiler, or contracting any of the passages ; it is better to let the heat diffuse itself fully throughout the entire heating surface. The shorter the steam is cut off in an engine, the more coal is saved ; providing the engine is large enough, and runs fast enough, and the cylinder and steam chest are protected by a perfect non-conductor. Cutting ofl' at one-third of the stroke the engine will give twice the power with the same fuel, and so on in the same proportion. There are two kinds of cut-oft's, the fixed and the variable which are regulated by the governor. Where the power required is variable, the latter one gives the best result, but where it is constant the fixed cut-off does equally as well. The grate surface for your boiler should be about 14 feet. J. W,, of Kansas.—The specimens sent are not aerolites, but simply chalk flints washed out from chalk beds. The white exterior which you think shows where the surface has been burned, is the hardened chalk,having a scored or indented surface produced by the washing away of the soft chalk. Chalk beds are composed of the calcereous or soft shells of marine animals ; the moss agates are also composed of shells of a harder nature, due to the presence of silex. Most chalk beds contain small masses of these silicious shells, which form isolated nodules of flint. They are picked up among the alluvial deposites, and go by the name of moss agates. J. 3r.,of N. J.—We are satisfied that all things considered, pine is the best timber for pump logs, wooden pumps, etc. Cucumber wood, would not be so likely to give the water a taste at first as pine, but whether this property is combined with power to resist decay to so great a degree as pine under the same circumstances can only be settled by experiment. J. L., of Pa.—According to Bernoulli, the pressure at which steam becomes water is 8,500 atmospheres, and the temperature 800 C. These figures are considered too small by some more modern investigators. Your last query is too indefinite. M. E. C, of Wis.—We infer from the piece of boiler you send us, that the iron was of bad quality, too thin, and that it had been overheated, either of which causes, or all combined would accouut for the explosion of your boiler. S. R. S., of La.—We have never published the machinery to which you refer for laying down rail, such as was employed on the Central Pacific Railroad. H. W., of N. I.-We do not know that dynamite is on sale in this city but you could ascertain by addressing Tal. P. Shaffner, New York city. J. N. C, of Ind.—A liquid blueing considerably used latterly is the soluble or basic Prussian blue, made by the action of ferrocyanide of potassium on a proto-salt of iron, and subsequent absorption of oxygen. You can get it from druggists. C. (x., of Ind.—The causes for the springing of a shaft are enumerated on page 243, current volume. The obvious remedy is the removal of those causes. J. W. S., of Me.—The heat of the electric light produced by two of Bunsen's elements,if proper adjustments are made with the apparatus used, ought to melt silver or copper. J. W. H., of Iowa.—Your, uery is baed on insufficient data. It oan not be answered witkoiiS, yi "8*ate.iat of strokes per miaute,' length of stroke, and area of piston, as well as pressure in boiler.