Correspondents who expect to receive answers to their letters must, in all sign their names. We have a right to know those u,ho seek in -formation 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 instruction of our readers, not for gratuitous replies to questions of a purely business or personal nature. We will publish such inquiries, however, wll.en paid for as advertisemets at $1*00 a line, under the head of »Business and Personal.» P-P-AUveference to back numbers should be by volume and page. J. R M., of Kansas.To find the flow of water through a ;3-inch orifice under a head of twenty-live feet, you must first determine the velocity of the flow per second, and multiply this by the area of the .aperture. You will then have the theoretical flow per second, although this is subject to some variations consequent upon the shape of the aperture, and other considerations which must be taken into account. Assuming that the aperture is round and the diameter two inches, the velocity would be forty feet per second. The area of the portis 314 square inches, which, multiplied into the velocity per second in inches, will give the amount theoretically discharged in cubic inches, or 1507-30 cubic inches per second. Two thirds this will be the actual/low, or 1004*8 cubic inches per second, equal to 1-66 horse.power. To utilize this power economically we advise the employment of a small turbine. A good work for you to get on such subjects is» Sillimans Physics.» G. 13. A., ot Ohio.Cotton cloth may be rendered nearly fire-proofby steeping it in a solution of alum and letting it dry. A better process is to starch It with starch mixed with phosphate of ammonia, a little more by weight of the salt than of the starch. Grind the dry starch and the salt together in a mortar, and then prepare the starch with the mixture in the usual way. After starching the cloth with this preparation, it should be rolled up in a dry cloth, and allowed to remain till nearly dry, and. then ironed, using a little white wax to prevent the sticking of the iron. H. 13., of Tenn.It takes just as much weight to pull down a balloon as it will carry up, and it is one of the most uneconomical of machines. It can only be advantageously employed where 11a other means of transportation are practicable. A balloon might be made to work in the manneryou specify, and from the novelty of the thing passengers might be attracted. You are under a mistake as to the use of chairs on railroads. A rail placed on a tie without a chair, would soon be jammed down into the wood under heavy work. You should see and talk with some experienced railroad engineer. F. J., of N. Y.There is no doubt that the diving dress used by divers in submarine work, would have enabled people to have descended into the Avondale coal mine without danger of suffocation ; but the dress is too weighty to be used in work unless partly sustained by the buoyant power of water. Besides the walls of a coal mine are very different things from water walls, and flexible pipes would stand a poor chance of maintaining their integrity in being sawed across their sharp angles. G. L. 13., of Mass.The products ot the combustion of all hydrocarbon oils are carbonic acid and Vi ate]-. The carbonic acid is formed by the chemical union of the carbon in the oil with the oxygen of the air, and the water is formed by the union or-the hydrogen in the oil with the oxygen of the air. Ordinarily, the water, being oonverted by the heat into steam, escapes notice ; but wlien a cold body, as a piece of iron, is held for a moment in the :Bame it condenses this steam and the water becomes visible. The theory of your friend is all wrong. C. P. S. W., of N. C.The white earth you send us is silicious lime, resfPiting from the remains of minute diatoms. Under the microscope the shells of the diatoms, covered with beautiful and delicate lines, are distinctly visible. 1Ve can have a sketch made of some of these shells, if you desire, at a charge of $5. The earth will probably be useful as a polishing: powder. G., of Tenn.The recipe for the hair composed of oxide of bismuth, spermaceti, and lard, recommended to you, will be as harmless as any other grease plaster provided tlie oxide of bismuth does not contain arsenic, withwhich it often is found mixed. As a hair renewer it is no better than barn yard manure or roadside mud. J. S. C., of Me.The sectional area of the horizontal flue leading from your boiler to the chimney, ought to be twenty-two inches in diameter instead of sixteen. No advantage would result from making the flues of chimneys taper towards the top. Horizontal flues ought to have from one fifth to one sixth more capacity than upright flues. A. W., of N. Y.We believe a fan to be a very uneconomical method of conveying the sawdust shavings, etc., from a mill to a fire room and cannot therefore advise it. We infer this from general principles, as we have not seen a fan used for that purpose. We are confident, however, that you will do better with the drag hitherto employed. J. R R, of Md.We think salt as good as anything to pack eggs in for winter use. They should be keptin a dry cool room but not wheretheywill freeze, and the package should be turned once a weeK to prevent the eggs from settling to one side of the shell. J. L. RNothing yet discovered is more effectual in retaining heat in vessels than thick coatings of loose felt. You can take a use-ill lesson from the Norwegian cooking apparatus, illustrated and described on page 161, current volume,of this paper. S. S., of Conn.You can use screws in making the model. The mineral you send appears to be mica schist, containing minute garnet specimens. J. H. Keine.We advise the use of plumbago (black lead) mixed with tallow for wooden cogs. W. E. E., of R I.Etherial phosphorus, so-called, is a simple sdtution ofphosphorus in ether. G. G. W., of Pa.The information you seek will shortly appear in our columns. tm\t ^nuxum ;uut Jmwp patents.