F. N. R, of 'Wis.—Copper is superior to iron wire as an electric conductor, because it offers less resistance to the passage of th2 force. The coarse copper wire in the the helix composed of several strands connected together. Iron wire is not suitable for a helix, because of its great resistance. Pure copper wire for making a ribbon coil is superior to tinned copper. The finer the wire, and the mOIe numerous the convolutions, the more powerful will be the electro-magnet. S. H. J., of Md.-In making surveys, you should do so upou'a meridian established by your own observations ; but in making reports of your labors you should state this clearly, so that the difference between your results and the map, where it is established authority, nay be made known. J. W. C., of N. Y.—We have received your sketch and description of a propeller to be operated by a reciprocating motion, and to open and dose like a duck's foot. This kind of propeller is older than the paddle wheel and wi\s invented by the Marquis of Worcester. You will find it illustrated in Vol. 5, Sci. AM. F. H. H., of N. Y.—The plau you propose to launch the u Leviathan' ' is simply a dry dock. It would have been more to the credit of her designer and builder had they built such a dock before commencing her construction ; but that is out of the question now. M. Brttnel undoubtedly expected he could accomplish the launch more easily. F. A. L., of N. Y.—As a substitute for coal tar to paint wooden posts and sills, where these enter the ground, use a paint compposed of boiled linseed oil, as-phaltum, and red lead ; its color will be deep brown. Introduce about three pounds of ground asphaltum into every gallon of oil, when boiling, and stir until it is dissolved, and then add red lead slowly, until the paint attains to the i-roper consistency for putting on with a bi-ush. This paint is a good protection, but not quite equal to coal tar; but it soon dries, and the posts can be handled freely, which is not the case with the tar, as it does not d -y, has a. very offensive odor, and a disagreeable appearance. II. H., of Pa.—The sample of mineral you sent us for examination al)pears to us to be a slaty shale ; we do not think there is much, if any, carbon in it. But this you can easily t,ry for yourself, by placing it on a fire, and if it only becomes r'3d-hot, without burning, you may be sure it is of EO use. It is rather like an inferior slate found in this State. E. D. 'r., of Ky.—Good concrete wall is made with common mortar and small stones. We cannot recommend such a wall for building purposes ; it answerarery well for some situations underground, in which cases hydraulic cement is employed for an outside coating. J. R., of N. Y.—An engine of 12-inch bore, 16-inch stroke, and making 130 revolutions per minute, will require more steam than one of 10-inch bore, and 24-inch stroke, making 80 revolutions per minute, but it will give out more power. 'Ve would prefer the engine with the proportions of 10-inch bore and 2-l-inch stroke, if it is of sufficient size to do your work. Its valves do not require to be opened and closed so frequently as the one which makes a greater number of revolutions. J. McM., of 'Ky.—A current of electricity does not produce an explosion when passing through free apace, or on a good conductor. Au exIJ osion by electricity is caused by the passage of a cunent through a resisting medium. You cannot forma vacuum behind and before the piston in a cylinder by alternately heating a piece of spongy platinum at each end with a galvanic current. An explosive engine, with the use of gun cotton as tho agent, is fur inferior to a steam engine in every sense. Do not waste time and money in such experiments. See our opinion given to another correspondent, on the same subject, in No. 18, this Vol., Soi. AM. J. P., ofLa.—A whitewash made by adding' to each bushel of lime, one pound of the sulphate of zinc is excellent for outhouses. F. B., of Ala.—Any overshot wheel made of wood will soon.go to wreck if the stream that drives it becomes dry during a part of each year. This is o ving to the nature oHhe materials of which it is made. Were it entirely constructed of iron, it would endure much longer, but it is more expensive. J. H. W., of Iowa.—The cheapest and best ice-house which you can erect ia simply a frame building, well boarded up inside and outside, in roof and sides, so as to leave a space between the boards, which should be atuffed with a good non-conductor, such as charcoal Qust, dry sawdust, or if you cannot obtain these, use straw. Build the house above ground, and with a drain under the floor. This plan is simple, and the cost will be small. E. R. B., of Conn.—In general a patent for a new article of manufacture covers the thing made as the specification describes. For example, a patent as a new article of manufacture for a pen-holder composed of gutta percha, would cover the making of pen-holders of the gum named. But it would not cover the making of any otier article, nor the use of any other material. The value of a patent of this character depends upon the nature of the invention. and upon the specification. In the supposed case, namely, mechanism wholly new, it might be better to claim all the new features separately. B. F. B" of Pa.—The best cement for iron and wood is a mixture of whiting, linseed oil, red lead and pounded glass whichforms a hard and durable cement. H., of N. Y.—There is no special rule for coloring mechanical drawings, as nearly every draughtsman employs some method ofhis own. All drawings should contain more or less color, and the idea is not peculiarly French. T. W. B., of Ohio.—We do not Consider it profitable to encumber our columns with abstruse mathematical 1 questions which should be learned at school. Onr objects are rather to eliminate the results of the sciences as applied to arts in general, without undertaking to instruct in the more elementary principles. It is an error to suppose that "the books" ' do not contain the information you seek to promulgate. In Davidson's great work, out of which our small treatises are made, every difficulty in mathematics is touched upon, and the Encyclopedia Metropolitana has a still more elaborate treatise covering the whole ground. A. M. G., of Phila.—Many tinsmiths make their own spelter, by using half lead and half block tin. Melt the lead first in an iron vessel, then add the tin. When the two metala are melted and mixed together, run them out into molds. Phelps, Dodge & Co., this city, are large importers ofspelter. J. W., of N. Y.—There is a boring tool which would answer your purpose, but we can only give you an idea of it by drawings. A., of R. I.—Nearly every sculptor uses a different clay for his modeling ; but we cannot recommend you to a better than to take fine powdered clay, mix with it a little whiting, and work to any consistency you wish. The best work on ana tomy, for young artists, is Wilson's; it is an English book, and published in this city by D. Appleton & Co. F. D., of N. J.—We cannot give you the inventor's name, nor tell you how the article referred to \j! prepared. Money received at the Scientific American Office on account of Patent Office business, for the week ending Saturday, January 9, 1858 :— J. L., of Tenn., $25 ; C. & M., of Iowa, $35 ; L. B. S., of Conn $25 ; W. H., of N. Y., $30 ; I. H., of N. Y., $30 ; G. S. C., ofIll., $30 ; II. R., ofInd., $30 ; A. L., of Conn., $30; C. V. W., of Masa., $30; H. & J. S. B. N., of Maine, $3; H. P., ot Pa., $55 ; J. K. B., of N. Y., $25 ; C. H. E., of Wis., $10 : J. A. & F. D., of N. Y., $30 ; W. & C., of Conn., $30; T. R., of Pa., $55; O. L. C oflll.. $115 ; M. V. A., of N. Y., $50 ; C. & Q., ofMaas., $25; H. A. W., of N. Y., $30; J. H., of Ill., $25 ; A. C. R., of Conn., $25 ; G. D., of Pa., $25 ; J. S., ofInd., $25. Specifications and drawings belonging to parties with the following initials have been forwarded to the Patent Office during the week ending Saturday, January 9, 1858 L. S. C., of N. Y. ; J. C., of lI:Iass.; C. C., of Pa. ; J. J. A., of N. C. ; J. L., of Tenn. ; E. L. E., of R. I. ; W. L. C., of Va.; R H., ofN. J. ; C. J. R, ofMd.; M. W., of N. Y. TO OUR SUBSCRIBERS. RECEiiTB—When money is paid at the office for subscriptions, a receipt for it will always be given ; but when subscribers remit their money by mail, they may consider the arrival of the first paper a bonafide acknowledgment of the receipt of their funds. The Post Office law does not allow publishers to enclose receipts in the paper. BACK NrMBERs.—After the 1st -of January, we shall commence sending the SCIENTIFIC AMERICAN to new subscriber's from the time their names are received, unless the person ordering expresses a desire to receive the back numbers, in which case complete sets will be furnished from the commencement of the volume. TERMS OF ADVERTISING. Twenty-five cents per line each insertion. We re-pectfully request that our patrons will make their advertisements as short as possible. Engravings cannot be admitted into the advertising columns. *.* AU advertisements must be paid for before inserting. WOODWORTH PLANING $ MACHINES— TT Having over $40,000 worth now completed, I will sell, from this time hencefortli, at a very reduced price, and am ready to constructaw BZL n l on hand at h rtn ice. JOHN H. M E, 57 Pearl st., Brooklyn, Long Island. CAWS-I WAEEANT MY SELF-STRAINING SSawt-leff B n uclit e aathe av r ge cular saws, to cut logs of the worst kind , as well as can be done by any sash saw, with more ease, and with two-thirds of the power, or no pay. My feed and gig works are equally good, and I am ready to " face the music.' S. E. PARSONS, Wilkesbarre, Pa. To MILL-O NE MILLERS, AND Mill-wrights in the South and Sourth-wst.— Having received the'Agencyfor the Southern States for an improved mrchant Flowing Mill and a Bolting Apparatus, made of gauze w e, together with the best Machine in use, orders are solicited for same. Address LANGLEY & CO., Patent Right Agents, care Mechanics' Institute, Nashville, Tenn. FOR SALE—A SECOND-HAND ENGINE, Boiler,$ Blowing Cylinders, suitable fOT a foundry or furnace. Cylinder, by 32 inc s, on iron bed. Boiler, 40 inches by 26 feet, double fluea. Three blowing cylinders, each 16-inch bore, 39-inch s oke, with all the casting, bolts, timbers, pipes, gearing, breeri , &c, nece ary to set running. Cost $1,600, and though nearly as good as new, will be sold for $1,200. We sell it fsmU, but simply because we haveno:e forit. A so one 8end -hande ine,wrr 10 by 30-inch cylinder. JAS. A. HAVEN & CO., 177 Second st., Cincinnati, O. ENGINEERING — THE UNDERSIGNED IS prepared to furnish specifications, estimates, plans in general or detail of steamships, steamboats, propellers, high and low essure engines, boiters and machinery of every description. Broker in steam vessels, machinery, boilers, &c. General Agent for Ashcroft's Steam and Vacuum Gages, Allen & Noyes' Metallic Self-adjusting Conical Packing, Faber's Water Gage, Sewell's Sonometers, Dudgeon's Hydraulic Lifting Press, Roebling Patent Wire pe for Hoisting and Steering purposes, Machin Oil of the most approved d, &c. CHARLij W. COPE ND, ulting En n r, 64 roac y, N w York. READ—NEW CATALOGUE, (FOURTH EDITION), with two hundred and fifty illustrations of Mathematical, Optical and Philosophical Instruments, and attachment of a large illustrated sheet, represent- t u en ua i d a w Il be vere p ic ion,to all i ar of e United States, by srading cents postage stamps or money, wh amount will be d from the bill, W an order ia s nt c: T. R, No. 635 Chestnut st., Philadelphia, Pa.
This article was originally published with the title "Correspondents"