COERESPONDENTS who expect to receive ansioers to their letters mttst,in all cases, sign their names. We have a right to know those who seek in-rormationfromus; beside, as sometimes happens, we may prefer to address correspondents by mail. PECIAL NOTE.—This column is designed for the gener al inter est and instruction of our readers,not for gratuitous replies to questions of a purely business or personal nature. We will publish such inquiries, however-ichenpaid for as advertisemets at $1-00 a Line, under the head of ''Business and Personal." 'SW'A.ll reference to back numbers should be by volume and page. L. M., of N. Y.—Lime in the form of mill?: of lime will precipitate the carbonate of lime from water which is hard owing to the presence of that salt. The reaction is the combination of the lime with the carbonic acid in the water, which enables the water to hold the carbonate of lime in golntion. If the water is allowed to stand long over precipitated carbonate of lime, it will, by the absorption of carbonic acid from the air, regain the power of dissolving the carbonate of lime which will render the water again hard. B. O., of La.—We much doubt whether the sinking of a water wheel made hollow and water-tight between the arms, into water, so that its weight will be supported by its buoyancy and relieve the journals from friction, will be productive of a saving in power. We should expect the friction of the periphery upon the water which will support the wheel, to be more than that upon the journals in the ordinary way. You need not sacrifice any head to try your experiment. All that is necessary is to sink the wheel pit. C. E., of Maine.—An illustration of your invention in the Scientific American could be obtained at less cost than an engraving done in inferior style and printed in circulars, which you would find it difficult to distribute judiciously in large quantities. This hint is a practical one, and worthy to be thought of. After we have printed an illustration we forward it to the patentee. S. T., of Miss.—The object of rifle grooves is simply to give a rotary motion to the ball on its axis lying in the path of its projection. It is not to retard the ball so thatr.the powder may exert greater force upon it before it leaves the gun. It has been found that the rotary motion thus imparted gives greater directness to the course of the ball, in other words, the ball will '' go straighter " to the mark. F. M. H., of N. Y.—We know nothing of tho engine about which you inquire. It will only be fair for you to say that your boiler saves one hundred per cent of fuel over tfle best boilers now in use, when it has been proved by actual test to do what you say. You will probably wait some time for such evidence,as such a saving is not theoretically possible and is practically impossible. II. C. S., of 111.—We know of no simple test that can readily ' be applied to the detection of cotton seed oil in linseed oil. It is diflicult to detect it with the best appliances known. The presence of lard oil, and similar adulterations, is best detected practically by the difficulty with which such oils dry. Linseed oil adulterated with lard oil will always be tacky when pure linseed oil has become hard and resinous. J. H. IL, of Va.—The ammonia prescribed as a remedy for toothache is the aqua ammonia of the shops. AVe advise you, if you are satisfied your neuralgia proceeds from decayed teeth, to have them extracted. In a personal experience which enables us to sympathize fully with you in your affliction, we have found that to be the only sure thing:. H. A. R. of Del.—The use of canned fruits and vegetables is constantly on the increase. We are informed that many manufacturers were unable to meet the demand for their goods last year. So you see that any improvement upon present processes in this industry, has a good chance for success. We are unable, however, to pronounce upon the value ot your apparatus without seeing its operation. Your description of it is not clear to us. V. C, of Pa.—We do not recognize any patentable features in your plan of steam engine. It seems to consist in a modification of the mechanism, and does not contain anything essentially new. The formlis a good one no doubt to economize space, but even in this respect it is no better than some others. E. B. J., of IU.—You are all wrong in your premises. It is the oxygen that is consumed, i. e., combined with carbon in the experiment you describe. The nitrogen is left. Oxygen does not support com-t)ustionby "mere presence." It unites chemicallyjiwith the substances burned. J. G., of Vt.—You are certainly an amateur, as you say, or you would know better than to use a file to finish apiece of metal in a lathe, which is required to be perfectly cylindrical. The turning tool should be the last tool to touch it. The best thing for you to do is to visit some machine shop and get some practical man to show you how to get a smooth finish without a file. li. S.,of Pa.—Hoe's rotary press was first used on the Philadelphia Public Ledger in 1847. The most important improvements in printing presses have been made within the lastthirtyyears. TheBullock press prints on both sides of the paper the paper being fed in from a roll, and cut off in sheets after printing. The effect of rollers upon type is more injurious than the pressure of a flat surface. J. C, McD., of Ca.—You are evidently confounding the terms "gather" and " set" as applied to wagon wheels. Gather" is the inclination ol the forward parts of the wheels towards each other, dependent upon the peculiar construction of the axletree. "Set" is the inclination of the bottoms of the wheels toward each other. Think the matter over again. C. K. H., of La.—The resistances of media upon bodies of equal size moving with equal velocity and of the same weight and form, are as the densities of the media. Water being 800 times as dense as air will offer 800 times the resistance of air. H. Ij. B , of N. Y.—We know of nothing better for polishing any kind of metal where a very high luster is required than rotten stone followed by Paris white and rouge. The latter was formerly very extensively usedf or polishing the silver plates for daguerreotypes. J. K. J., of N. J.—The best qualities of chrome iron ore contain sometimes as high as sixty per cent of the oxide of chromium. We doubt whether you have found it as you seem to think, although it is possible. So far as we know it is only found in serpentine rocks, either in veins, masses, or pockets. E. B., of Mass.—The connecting of your stove pipe with two chimneys will not avail o relieve you of the too powerful draft, unless one of the chimneys is much lower than the other. The answers to your other inquiries willbe found in an article entitled "Hints on the Burning of Anthracite," which will shortly be published. R. M. N., of Ga.—Mineral paints are for the most part oxides and sulphides of metals. The ochers about which you particularly inquire, are mixtures of the oxides of aluminum and the hydrated peroxide of iron, with, in some instances, oxide of manganese. P. McC, of Cal.—There is plenty of room yet for new ma chines which will manufacture a good quality of wood pulp. To pay well they need only equal in efficiency some already in use. Should your machine prove to be superior to those, success awaits it. Actual experiment can only decide the question. Petroleum Broker is reminded that in order to get his case before us properly he must put in an open appearance. Weare not in the habit of wasting our time upon anonymous correspondents.