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HINTS TO CORRESPONDENTS. Names and Address must accompany all letters, or no attention will be paid thereto. This is for onr information, and not for pnblication. Reference to former articles or an"wers should give date of paper and page or number' of question. (nqulrle" not answered in reasonable time should be repeated; correspondents will bear in mind that some answers require not a little researCh, and, though we endeavor to reply to all, either by letter or in this department, each mnst take his turn. Special Written Information on matters of personal rather than general interest cannot be expected without remnneration. Scien tide American Snl'plement" referred to may be had at the office. Price 10 cents each. Books referred to promptly supplied on receipt of prIce. Minerals sent !kd examination shonld be distinctly marked or labeled. (1) G. A. asks for a receipt how to make waterproof writing ink, an ink which will not blur if the writing is exposed to rain. A: Dissolve 2 ounces sheUac in 1 pint alcohol (95 per cent). filter through chalk, and mix with best lampblack. (2) D. H. A. asks : What is the best known substance or preparation that will waterproof canvas and make it mildewproof and proof against rot ? What is the best known waterproof cement for canvas, that will make sewed seams in canvas water proof? A. We recommend paraffine applied to the per fectly dry tissue and melted in with a hot sadiron, for both purposes, or 1 pint linseed oil and 1 ounce beeswax may be mixed and applied with a brush and allowed to dry before use. (3) C. S.Any conductor, carbon or metal, is heated by theelectric current. Lime is a non condnctor and will not be heated. (4) W. B. asks what ingredients42 2'6 in dissolving gum shellac in water. A. Borax. v (5) W. W. W. seeks information as tobri the best methods of making magic lantern slides from dry plate negatives. By what is known as the " wet plate process " very good slides are easily made, but it is somewhat messy and is productive of blackened fingers. The glass plate, after being thoroughly cleaned, is coated with collodion, which can be bought ready made from dealers in photographic supplies, and next sensitized by dipping for a minnte or two in nitrate of silver bath prepared as follows: NItrate of silver................,........437 grs. Distilled water......................... 16 oz. Nitric acid..".......................... 3 drops. Saturate the solution with iodide of silver and then filter. If the bath is milky, set the bottle in the sun for two days, until the solntion clears up. Then decant, and it will be ready for use. After sensitiziIlg,the plate, while wet is at once exposed in the camera, then removed from the plate holder aud immediately devel. oped with the following solution : Protosulphate of iron................... 1% oz. Nitrate of baryta...................... 1 oz. Water..................................16 oz. Alcohol................................ 1 oz. Nitric acid...........................40 drops. After disoolving, filter out the white deposit and keep well corked. When developed, it is washed under the tap and fixed in a solution either of: Cyanide of potassium............,.......30 grs. Water.................................. 1 oz. or Hyposulphite of soda......... .........50 grs. Water................................... 1 oz. The cyanide of potassium is a deadly poison, and is only advantageous to employ when it is desired to tone the slide to a blue color soon after lixing. When hyposulphite of soda is used, the plate should be thoroughly washed under the tap for two or three minutes. After fixing, tbe slide may be toned to a brown color by a so lution of chloride of gold 1 gr., dissolved in 10 oz. of water, or instead a purple color, by a solution of 4 grain of bichloride of palladium to one ounce of water, in a clean porcelain dish or tray. The sOlntion should be strawberry in color. The plate should be left in thf f tray until the film assumes a black color on both sides when looked at byreflected light. The tones are more easily obtained when the slide is fixed with cyanide of potassium. Very good slow gelatino.bromide plates cau be pnrchased, known as lantern or trausparency plates. ~ These may be exposed in the camera, or by contact witb " a negative. Then they are developed in solu.tion of a hydrochinon 15 grains, snlphite sodium 50 grs., water '" 1% ozs. carbonate of potassium 30 grs., and fixed in a - solution of hyposulphite of soda 1. oz. to 8 of water. e Several plates, 4 or 6, may be developed at one time in n this developer, and are then removed as fast as deve !" loped. ~ (6) St. M. I. asks (1) for a simple methoc ,B of purifying and decolorizing crude bitartrate of po K tassium as obtained from wine casks. I have a cer , tain quantity on hand and would like to decolorize it Indicate any method, but the simplest is preferable. A. The tartar, previously pulverized, is boiled with wate in copper boilers. The solution when saturated IS trans f erred to earthen pans, wbere it deposits on cooling a crystalline layer, nearly free from color. This is re ~' dissolved in boiling water, and the solution, baving '. been mixed with 4 or 5 per cent of pipe clay, is evapor ated to a pellicle. The clay precipitates with the co ' oring matter, and the clear solution, as it cools, depOSIts white crystals in crnsts, which, upon being exposed .t * the air on linen for several days, acquire an increased degree of whiteness. These constitute the crystals tartar of pharmacy. 2. We find some difficulty in ve work'mg the Toepler-Holtz f'rICt'IOnaI machine, and ar unable to place the cause; for, ayearago,it worked af te turning but a few times, and now it. requires quite length of time to charge it. If the plates are to be var. nished, please indicate how to prepare the varuish. Wipe off with a warm dry cloth all part of the ma- chine. Dust ae well ae moisture interferes with Its a?tion. The varnish for the plates is alcoholic solu g tlOn?f shellac. 3. How many 1 gallon Bunsen cells are reqUIred to run two sIx-candie power lamps ? A. About ten cella. (7) W. J. M. asks: 1. What is the vulcamzed fiber washer that clamps the glass disks to the bosses in a Wimshurst influence machine composed of, and how made? A. They are made of India rubber, sulphur. and other substances. You'can buy it in large or small pieces of dealers in electrical supplies. 2. I have a Holtz machine, and the amalgam In using is mercury, tin, and zinc. I am told there is a better amal gam which is of an old gold color. Can you tell me how to make it? A. It is bisulphide of tin, the old aurum mUBaivum, formed by sublimation from an amalgam of tin 1 part, mercury 6 parts. Of this amalgam 18 parts are mixed with 6 parts salammoniac and 7 parts sulphur. The bisulphide of tin remains behind in the retort. No amalgam should be used on a Holtz machine. 3. Are dry cells which are made in Germany equal to the Leclanche cell ? Can you inform me what they are composed of ? A. Dry cells are of rather high resistance. Gelatin or agar.agar jelly is used in them instead of water. 4. What chemical is it you apply to a file so you can file glass? A. Turpentine and camphor, or simple water. We can give no information as to the gas lighter you speskof. (8) J. H. K. asks : 1. Will the motor de- scribed in Scientific American, March 17, run an ordinary tricycle and about what speed? A. It should develop a speed of abont 8 miles an hour. The trouble would be to get a compact and light battery. 2. Is there any good paste to make paper adhere to a brick wall exposed to the weather ? A. Shellac is the best we can recommend, and that is imperfect. Tack muslin over the wall and stick paper to that. 3. I have read with a great deal of interest the articles on speed of railroad trains, and would like to know the fastest time ever made. and where and when? A. On the New York,West Shore and Buffalo Railroad,betweenChurch-ville and Geneva Junction, on July 9, 18P5, a special train attained a speed of 87 miles per hour. It rau 422'6 miles in 9 hours and 23 minutes. (9) D. J. B. You cannot keep the s bright color of polished iron on the hot parts of an en' gine withont constant attention and wipiug with engine 0.i1. Oxalic acid may help the cleaning, but the acid left on the bright surface favors oxidation. For cleaning. use tripoli, rotten stone, or pulverized pumice stone, with engine or kerosene oil. Neglected or dirty spots may be removed with a scraper and fine emery paper, and afterward rnbbed with oil. Every part of bright work around au engine should be wiped with oil. Moisture immediately discolors a clean bright surface. Polish the lubricator with rotten stone and oil only, and only when necessary. Too much polishing soon makes it look old from wear. (10) G. asks how to make the preparation whereby solder will readily affix itself to other , metal than tin; for instance, brass wire, etc. A. Use a solution of chloride of zinc and chloride of ammoninm, or what is called tinner's acid, which you can make by dissolving zinc in hydrochloric acid to saturation. Add 10 to 20 per cent water and as much salammoniac as the bulk of zinc dissolved. (11) C. F. R. asks a formula forgiving a green color to finished steel. A. We know of none that will wear well. The sulphide of lead process gives a film of var.iegated colors, which may be covered with ' lacquer. The article must be perfectly clean, with a s dead linish, then dip in a solution of V& ounces hypo sulphite soda in 1 pint water,in porcelain dish, to which add la ounces acetate of lead previonsly dissolved in 1 pint of water. Mix and heat the whole to nearly boil, ing. 300. Boil the article in caustic soda and water strong enough to clear of grease or finger marks, rmoe in boiling water. Dip in the hot solution and examine 3 until the desired color is obtained,which may be seconds J or minutes. The effect runs through several colors in ' succession. r (12) Yacht.The determination of the . draught lines of a yacht is not an easy matter for an f amateur. The weight of all the materials composing j the hull, risging. ballast, and furnitnre of the yacht mnst be computed from the details of the design, and I the displacement of an equal weight of water computed e within the lines of the boat. For the details of con struction we refer you to Scientific American Sup e plement, Nos. 42 and 67. also " Model Yachts aud f Model Yacnt Sailing," by Walton, which we can mai j for 1.85. s. (13) P. H. G. asks (1) why steam ex h hausted into the atmosphere on a cold day lasts much f longer than the same amount exhausted on a hot sum r mer's day. A. In a hot, dry air the steam is instantly a absorbed by the air and becomes transparent. warm, er. dry air having a large capacity for holding moistnre. On the contrary. cold, moist air may be saturated with - water and has little or no capacity for further absorp. tion. Then the exhanst floats away in clouds. i j What is the cause of that peculiar roar which of ten issues from elevated road locomotives ? A. The roa ' you refer to is probably that caused by thevibration c the safety valve in blowing oft. As it is held by spring only, it at times vibrates or cbatters when th \' steam pressure is just enough to barely lift the valve. a- ater V. M. C. writes : 1. Can you de a scrie any process by which drawing paper can be mad re- pefectly transparent temporarily, that is, to have i g me back to its regular appearance and co.dition agai r- after the process, when required ? A. Treat with castor \/ oil. and it WIll be transparent; lafterward dissolve ou ts the oil with alcohol. 2. Can you describe something t harden crayon pencil and charcoal drawings an d .ketches so that they will not fade soon or rub off: some- of thing easily applied and able to give satisfaction i general ? A. Lay paper in a shallow dish, and pou re skimmed milk over it. When well wet all over. raise int er a vertical position and allow it to drain, removing wit a a feather the last drops from bottom edge. Dry car r. fnlly. Or wash it over with warm starch solution. thi A. isinglass water, or rice water, applying it with a broa a camel's hair brash. (15) J. D. B. and E. R. C. ask: 1. The ingradients, and their proportion, for the best cement to attach bicycle tires to their wheels ? A. You can make a very strong and tough cement by dissolving 1 part pure India rubber in 12 parts benzine, then adding 30 parts shellac, and heat carefully away from fire until the shellac is dissolved and the benzine has evaporated. 2. Is there any work published treating of the manufacture of bicycles ? A. No, except as you will see the different styles described in trade catalogues. 3. Is there a work on japanning or enameling, giving description of best arraDgement of drying ovens, etc. ? A. You will find an article on japans and japanning in Scientific American Supplement, No. 316. Also see "Workshop Receipts," third series, which we mail for 2. (16) B. M. P.Tell your friend the finest stationary engines made in the world, for economy, durability, and elegance iu design, are made in the United States of America. English engines are often bulky and clumsy. French engines are freqnently erratic in design and fragile in construction. (17) D. M. M. asks if there is any way to extract the oil from lamb's wool in alcohol. What proportion of alcohol to wool should be used ? Will the result besolid like lard or look more.like the alcohol ? I have tried the experiment of boiling both together, bnt it does not seem to change either. Perhaps you will tell me why. A. For extraction of oil v*e should ad vise the use of ether and a continuous fat extractor. On exposing the ethereal solution to the open air, the ether will spontaneously evaporate, leaving the oil. We doubt if you extracted much with alcohol, as the latter is readily diluted by any water in the wool. Snch an ex tractor as we refer to is described in Scientific Ameri can Supplement, No. 628. The oil will probably be yellow and thick as butter. (18) D. T. S. asks : 1. Whether or not there is any way of preserving (permanently) the beautiful polish of which copper is susceptible. A. Var nishing with shellac while the metal is warm and abso lutely free from the least particle of grease is a good method. 2. Can you give recipes for making gold and silver inks ' A. Gold or silver leaf are rubbed up with honey and dllnted with water. (19) A. G. B. asks how to make cake stencil ink. and nsed by wetting brush 'with water. A. Mix and dissolve hot, I part gelatine in 1 part water, and add and stir in sufficient coloring matter; 3 parts lampblack and a little indigo is a good mixture. To above 1 to 2 parts of glycerine may be added to mske it softer. (20) C. I. M. asks for a receipt for a stove blacking. A. 2 parts copperas, 1 part boneblack, 1 part black lead, mixed to consistency of cream with water. Two applications are recommended. (21) C. A. B. writes : I should like to know the most simple method of accurately determin ing the amount of tannin in the various tan liquors used in tanning leathers ? A. The determination of tanuin is sometimes attempted by the use of the hydrometer. The only correct methods reqnire considerable skill in chemical analysis. Sometimes pieces of raw hide are weighed dry, soaked in the solution, dried, and weighed, and the increase iu weight is called tanuin. This is not a very accurate method. (22) S. C.-The outside of finished bells are turned at a slow speed with very hard tools or scrapers. Dipping the castings in water before they are cold (300 to 400 temperature) will throw off the sand. If the metal has been poured very hot and the sand bnrnt in, an acid bath may be nsed, of nitric acid 1 part, water 4 parts. (23) P. P. D.Steam and the water in the boiler at 95 pounds pressure both have a temperature of 320 Fah. At 125 pounds pressure the temperature of both will be 352" Fah. Water can be heated to any temperature by confining it. (24) W. H. G.If you cannot wash off the fly specks with soap and warm water on a cloth, there is no way that an amateur can refinish lamp work with any satisfaction. To do this, the lamp must be taken apart and the brass work boiled in caustic soda to remove all oil and varnish; then rinse in hot water and dip in strong nitric acid for a few.. seconds only, when it will come out clean and bright. then rinse clean in boiling water. Dry m sawdust, brush off, and lacquer with' thin shellac varnish. The metal must be warm and perfectly free from grease. (25) K. K. W.The rust on the inside of yonr shot gun can be removed by rubbing with a perforated cork glued on a stick of wood. The cork is to be trimmed with a sharp knife to fit tightly in the bar reI. Then with oil and gronnd pumice stone. or tripoli, polish the inside of the bavrel until it is free from rust. (26) H. S.A little pulverized sal-am- moniacsprinkled on tin will make itflowfreeandclear. There Is nothing but an alloy of other metal that will jiake it melt at less than its normal temperature. (27) S. C.The best covering for pulleys is leather. Soughen the pulley with an old file. Use .he best glue, with its dry weight of glycerine made in the ordinary way. Glue the leather to the iron pnlley and lap from two to three Inches. Tough paste board does well while it lasts. Leather is the cheapest by its durability. (28) H. C. J. asks whether it requires greater velocity for arifleball to go through two 1 inch boards naIled together or through one 2 inch board. A. The penetration of shot is favored by a division or lamination of the resisting medinm. The two 1 inch boards together require the least velocity for perforation. (29) G. B. C. asks: Can you tell me of any filter or material fot one that will filter lime water and remove every trace of lime from it, leaving it pure? A. Lime cannot always be removed from water by filtratton. Sometimes it has to be boiled to secnre precipitation, sometimes It cannot be practically removed by any means. It all depends on the form in which it is present. If once precipitated, any good filter will reo move It. 347 (30) . D. B. asks: 1. How to stain a sole leather bag somewhat abraded a dark mahogany color. A. Mix 15 grains alkanet root. 30 gram. aloes, 30 grains dragon's blood, all in powder, with 500 grains alcohol (95 per cent). Moisten the bag with dilute nitric acid (1 acid to 5 water by volume) and then apply above solution. Rcpeat until dark enough. 2. I wish to calculate the power of a flow of water which flows in a broad, thin stream over a dam. A. Determine the " head" measured from level of main body of water above the dam to the level of the top of the dam, then multiply length of dam by 333, aud multiply this by the square root of the cube of the head expres.ed in feet and decimals. The result will be in cubic feet. See Trautwines " Engineer's Pocket Book," page 865. 3. What is the resistance approximately of carbon in sectional area snch as is used in arc lamps and how to calculate the resistance at different temperatures? A. Carre's circular carbons 1 millimeter (0'04 inch) dia meter have 50 ohms resistance per meter (3937 inche8). The resistance diminishes as the temperature increases. Bctween 0 and 100 C. the coefficient of reduction is )1912. This you can easily reduce to Fahrenheit's scale. (3t) T. A. R.There is no way to bend wood better or cheaper than by steaming. Gun stocks, if bent at all, are steamed and bent in the rongh. The trimmings are put on after the stock is finished. (32) M. F. S. asks how to solve the following questions: No. 1. Aclerk spends 20 per cent of 66% per cen t more than 14 of his salary, and thereby saves 533. What is his salary? No. 2. A cabinet maker dirccted his salesman to mark a set of furniture so that, by allowing 30 per cent on the marked price, he would realize a gain of 25 per cent. The sale.man marked the set by mistske 300, or at a loss to the dealer of 20 per cent of the sale. At how much less than the required marking price were the good.Annuncia marked by mistake? A. Solve No. 1 by following equation: I,et a: = salary; then: X (%Xi + %)x = S33 X = 1,453.63. Solve No. 2 by following equations: I,et = proper marking price, then y = cost. x ix. (1)y = x-ix-------------- 5 2oo_a|a (8)ai = 300 -------------= 192 5 X = 300. Or by analy.i. they may thus be solved: 1. He spends 66% 14 of his salary plus----of his salary, and what is left 5 amounts to 533. Taking it by percentages, what he spends reduces to 50 per cent + 1333 + per cent = 61'33+per cent. Subtracting this from 100 giveS 36'66 + per cent of his salary left unspent, which is equal to 533 533. His full oalary, therefore, is equal to ------------ 36'66 X 100 or 1,453.90 within limits of errors due to cautinued fractions. This is one .olution, but tbe problem is worded so badly that several meanings may be drawn from it. 2. The suit marked at 200 was sold at 200 less 20 per cent, or 160. Thi. gave a loss of 160 X 20 per cent or 32. Thus the good. cost 192. They were to be marked so that the selling price should give a profit of 25 per cent; 192 X 25 per cent gives 48. The .elling price shouid have been, therefore, 240. To this 25 per cent must be added for the marking price. or 240 X 25 per cent, giving 60. The marking price should have been 300. (33) Benzine asks : 1. How can I color a solution of rubber in benzine, black and brown? A. Use asphalt or coal tar. 2. Does dragon'. blood dissolve in benzine? A. Yes. 3. Would be glad to know through your valuable paper of a few colors that dissolve in benzine. A. Alkanet root extracts are peculiarly availahle for coloring benzine. (34) L. M. R. writes.: I have an incomplete Bunsen :6 by 8 Inch battery, a zinc plate 24 in. wide being substituted for the zinc cylinder. Will you please inform me how much and what kind of acid I must use to complete the battery? A. For porons cup nse following solution: Mix 14 part by weight of sui phnric acid with 3 parts water carefully and allow to cool. Dissolve six parts by weight bichromate of pots.h in 16 parts by weight of water by boiling, when cool mix two solutions and stir well. Use when cold. For glass jar use water. (35) C. H. asks: How to transfer a printed illustration on paper to a glass magic lantern slide. 'A. Soak the print in soft water, varnish or float the glass plate with dammar varnish or Canada balsam. Allow to nearly dry; when still tacky carefully press the wet print upon it, and let all dry. Then with a wet finger you can rub off the paper, leaving the ink. A second coat of varnish will improve it. (36) W. McP. asks: 1. Before a rain, the atmosphere contains moisture. The atmosphere and moisture weigh more than the atmosphere alone. Why then does the barometer not rise instead of falling? A. A falling barometer indicates the center of a storm disturbance, a rising one indicates its margin. These changes correspond with the rotary movements of the air, and not to the presence or absence of watery vapor in the air. 2. Can an electromagnet be constructed in the form of a ring? If so, what parts correspond to the poles ? A. It can be magnetized so that one end of a given diameter will be north and the other end south, or it may be magnetized so that it will exhihit no polarity until broken, when the ends will become magnetic poles. 3. Are street cars propelled by electriCity a success? If so, why are they not mOre generally used? If not, where does the tronble lie? A. Yes; they are being rapidly introduced all over the country. 4. How fine a wire must be nsed in the .econdary coil of an in dnction coil? A. No. 36. For induction coil constru.ction we refer you to our Supplement, No. 160, which we cansend yon for ten cents. (37) J. C. K. asks the most economical and most effective ..pomt at which to set compound I engines. where there is a coupl iug between them. to get best results. A. Set the cranks at right angles
