W. M., of N. Y.—A good steam gage should indicate the exact pressure per square inch in the boiler. The amount of steam at 212 which steam at higher temperature would make, may be at once computed from the pressure and the space itoccupies. For formulas to do this wereferyouto Bourne's " Cateehiam of the Steam Engine," page 85. What is meant by vacuum is the absence of any material thing. When two boilers are connected by a pipe of sufficient size, the pressure per square inch will be practically the same in each. Tiiis is what the gage indicates, not the sum of the pressures per square inch in each. F. S., of Pa.—Formic acid Was first found in the bodies of ants. It may be, however, prepared artificially. It takes its name from the Latin Formica, an ant. It may be made by'distilling with care in a retort, ten parts of starch, by weight, with thirty-seven parts of peroxide of manganese, and oil of vitriol and water, each thirty parts. There are many other methods of producing this acid, but the method given can be employed on a small scale in a retort capable of holding ten or twelve times ae much in the bulk of the mixture, as the latter is liable to froth when the heat is first applied. E. W., of Ohio.—The electric light is an expensive one, requiring a very strong battery. The calcium light will answer your purpose better and will be much cheaper for magic lantern purposes. You will find the subject of Taxidermy treated in any good encyclopedia. You will find the smell of your stuffed pole cat so persistant as to probably spoil itfor the cabinet. One of these animals ought not to be killed by violence if intended for stuffing. It is better to persuade him to contribute to science by a little arsenic adroitly secreted in an egg. J. V. R., of ?. Y.—To make soluble glass in small quantities, fuse together in a Hessian crucible, one part of clean sand or finely pulverized quartz, and two parts of dried carbonate of soda. When the fusion is perfect, pour out the mass on a stone slab to cool. Pulverize and treat with boiling water until all but the impurities are dissolved, then concentrate by evaporation in a porcelain capsule. ?. B., of Mass.—Portland cement concrete might doubtless be used to advantage in filling in the walls of frame buildings. In a building, the walls of which are already plastered, it would, however, require to be done with great care not to press off the plastering. To attempt to All ten feet in depth at once would certainly destroy the walls. Mot more than one foot could, in our opinion, be safely ventured upon. G. L. P., of Saltillo, Mexico.—The sketch you send us of an instrument for measuring the flow of streams at different depths ia simply a re-invention of an instrument devised by Pictot, and well known to hy draulic engineers in the United States and Europe. It is a good instrument, and that you should have reinvented it in the absence of any previous knowledge of it is creditable to your inventive talent. J. Y., of ?. Y.—Your request for us to furnish you with a design (for it amounts to that) for engines, screw, etc., for a boat 23 feet in length and 7 feet in width, designed to have a maximum speed of 8 miles per hour, would, if complied with, cost you about one hundred and fifty dollars. We prefer that some of the mechanical engineers whose advertisements appear in our columns should get this job. R. D., of ?. Y.—The recipe for hardening mill-picks to which you refer was as follows : Two gallons rain water, one ounce corrosive sublimate,one ounce sal ammoniac, one ounce saltpeter, one and one half pints rocksalt. The pieks shouldbeheated to cherry red, hardened in the bath, and drawn to temper. M. H. K., of Kan.—We do not know of any monograph on the sub ject of starch manufacture. If you could get access to *'Muspratt's Chemistry" in some public library, you would And; the subject very'fully treated. It is also discussed at length in Dr. Ure's *' Dictionary of Arts, Manufactures, and Mines. R. ?., of Pa.—We were not aware, till you informed us, that practical railroad men suppose the gravity of a train to be increased with its velocity, but we were well aware that what is called its " vis viva," increases as the square of its velocity. Is not that what you mean ? R. M. F., of Texas.—Capt. John Ericsson resides at 36 Beach street, New York. The other information you seek can be found in Ap-pleton'sEncyclopedia. Article" Oysters." E. T. D., of Pa.—You may remove the oxide of copper from the surface of that metal by ammonia in case you do not wish to employ acids. J. L. B., of Cal.—A patent can be obtained on your invention if it is what you state. We do not deal in patents and therefore must decline your proposition. S. MM of Mick—Your " new thing" in paper boxes is a good thing, and we think the claims are clearly patentable. S. B., of 111.—A liquid black varnish for stoves is sold in this market, and we presume yu can find it in Chicago.