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 vrefer to address correspondents by mail. SPECIAL NOTE.—This column is designed for the general interest and instruction of our readers,not for gratuitous replies to questions of a pureli business or per.sonal nature. We will publish such inquiries, however-when paid for as aduerliumete at, $l00 a line, under ike head of Bust ness and Personal. IWAtlreference to back numbers should be by volume andpage. S. J. K., of Vt.—No perceptible effect is produced upon a weight suspended from a spring balance during the conjunction of the sun and moon, by their united attraction. It is only upon very large bodies that this cause could produce a perceptible effect, and the effects perceptible in large bodies bodies of water are very slight ivhen compared to their entire bulk. When you consider that the highest mountains are far less in proportion to the entire bulk of the earth than the wrinkles on the rind of an orangeareto its bulk,youwill see that the highest tidal wave is a very small thing compared to the mass of the earth. The statement that the sun is four millions of miles nearer than was originally computed from the transit of Venus, is based upon more recent calculations made from other data and is now universally admitted by astronomers. Nitro glycerin is made by dropping glycerin into equal parts of strong nitric and sulphuric acids; it is a dangerous plaything. You will find some hypotheses in works on physics, upon the cause of the refraction of light, but nothing positive has been demonstrated as to its ultimate cause. H. A., of Col.—The cubic foot of water weighs 62J lbs. and converted into steam of atmospheric pressure would contain for each lb. 537 centigrade units of heat, and for the 62i lbs., 537x62i or 33,562 units of heat. A nominal horse power, per hour of 60 minutes, is obtained by the combustion of 1% lbs. of coal and evaporation of 25 lbs. of water, containing 25x537 or 13,425 units of heat. A nominal horse power perhour is equal to 33,000x60 minutes or 1,980,000 foot lbs. per hour. Dividing 1,980,000 foot lbs. by 13,425 would give us 1,474 foot lbs. as the mechanical equivalent of 1 unit of heat of atmospheric pressure—a portion of heat being lost. This equivalent is however too low by from 20 to 30 per cent. T. J., of 111.— Can two pieces of flat iron be so fitted with a file, that by placing one upon the other—no oil or other substance between—the atmospheric pressure will keep them together. We see no reason why the two pieces cannot be fitted by the file; it is often done by scraping surfaces. We doubt, however, if the surfaces are held in contact by atmospheric pressure alone. Cohesion of particles is probably the source of the force or attraction that holds the two surfaces together. A. T. C, of N. Y.—Castor oil is unfit for lubricating either gun locks or any other mechanism. It is viscid and gummy. Better is pure sperm, porpoise, olive, or poppy oil. Dont gum np your gun with it. G. F. S., ot Mass.—If you cannot reach the defect in your steam cylinder to plug it, or melt in a composition, stop the hole with a mixture of two parts sal ammoniac with eight parts fine cast iron filings. No sulphur to be used. A. T. C, ofTo remedy the fault in your blackboard give it a coat of lampblack and japan varnish. The lampblack should be deprived of oil if necessery by heating in the usual way. H. D. D., of Texas.—Water might without doubt be brought over the neck of land of which you speak in syphons. The distance however, is so great in proportion to the fall, that with your low head, you would get a sluggish fiow on account of friction, we do not feel justified in recommending the attempt as a means of getting power. A. W., of Mo.—The separation of silver from lead is profitably done in many places. The information you seek can be found in Phillips Mining and Metallurgy of Gold and Silver, sold by D. Van Nostrand of New York city. W. S., of Wis.—We have carefully examined your article upon velocity, and it is evident to us that you do not understand the subject. We prefernot to publish crude notions upon abstract scientific subjects. F. S., of Mich.—The cost of beet root sugar machinery given in our paper is based upon the charges in gold for such machinery in France. The other information that you seek on the subject will be given in the remaining article of the series. R. M. C, of Iowa.—We cannot undertake to verify the correctness of your analysis of composite numbers. Such investigations re quire more of ourvaluable time than we can afford to bestow upon sul) jects of that character. H. H., of Berlin.—You mistake our position entirely. We are opposed to all swindling rings, but we do not propose to revive and denounce old swindles that have gone out of sight. Our paper would not be large enough to keep pace with active operations of that kind going on. J. B. IT., of Md.—Wedid not preserve the letter of the correspondent to which you refer, therefore cannot send his address. E. jj.,-of N. Y.—Your idea of crossing Broadway by the use of wings or the flying trapeze, is novel, to say the least, but it scarcely merits notice as a practical scheme. E. P., of N. Y.—We recommend you to write to H. C.Baird, of Philadelphia, for a work on the art of perfumery. E. H., of 111.—The law enacted by the legislature of your State, in reference to the sale of patent rights, is intended to put a check upon frauds. It cannot interfere with legitimate sales-of patents. C. F., of Ohio.—Good white lead putty is as good as anything for an aquarium. The water ought to be changed several times however before putting in your fish. E. E., Ind.—A good varnish to protect tin plate is lacquer, similar to that used for brass, and applied in the same way.