Hints to Correspondents Names and Address must accompany all letters, or no attention will be paid thereto. This is for our information, and not for publication. References to former articles or answers should give date of paper and page or number of question. Inquirie8 not answered m reasonable time should be repeated; correspondents will bear in mind that some answers require not a little research, and, though we endeavor to to either by letter or in this department, each must take his turn. Special Written Information on matters of personal rather than general interest cannot be expected without remuneration. Scientific American Supplements referred to may be had at the office. Price 10 cents each. Hooks referred to promptly supplied on receipt of price. Minerals sent for examination should be distinctly marked or labeled. (1)C. E. De P. asks : 1. How many Edison lamps will the dynamo described in SUPPLEMENT, No. 161, run? A. One or two very small incandescent lamps. 2. If I were to make a machine double; the size of the one described, should I use the same sizes of wire, and how much, and would it supply as many lamps as the first one? A. In a general way, use wire of double diameter and six to eight times the weight. It would require three or four times the power to run it, and would supply three or four times as many lamps. 3. How large a machine would it take to run six 16 candle lamps, and how much power would it require? In making a machine for this purpose, is there some better form of armature and commutator which I might use, and would I want to use the same sizes of wire as on the other machines? A. It would take about 1 horse power. It would not be advisable to construct so large a machine on the plan given. The drum armature (Siemens) is preferable. We cannot prescribe the exact size of wire, as it varies with the proportions of the machine. (2)J. D. asks : How is dynamite that is used in the present time made? A. By mixing infusorial earth with nitro-glycerine. A recent proportion is 1 of carth to 3 of nitro-glycerine. (3)G. J. asks how he can find any number of points in a circle without going around the circle with compass. A. Using the radius of a circle as chord, the circumference can be divided into six parts. This gives three parts, and by halving the sides, twelve; then by taking three sides of the dodecagon at once, it gives four parts; by doubling the dodecagon it gives'24 parts, etc. But for most of the ordinary cases, the tentative method is most available. (4)L. E. C. asks (1) why secondary wire (of induction coil described in SUPPLEMBNT, No. 160) is wound in two sections. with insulated wire drum between them. A. To more perfectly insulate from each other members ofj the coil possessing great difference of potential. 2. If No. 28 wire would not be better for medical purposes? A. We would not advise you to depart from proportions given. They have proved very good. Almost any proportion of parts wiU answer for a medical coil. (5)A. T. G. asks: 1. Is 16 pounds of ice plad in a refrigerator every other day more serviceable than 8 pounds placed in it daily? A. The sixteen pounds would be the better if the refrigerator was not to be opened; normally, there would be no difference between them. (6)E. J.—To paint on glass, take clear resin 1 ounce; melt In an iron vessel, let cool a little, but not harden; then add oil of turpentine sufficient to keep it in a. liquid . state. When cold. use it with colors ground In oil.-The following is a receipt JUNE 19, 1886.] Sfdtutiiit %miitM. 395 for a liquid which will remove ink from paper: Take of chloride of lime I pound, thoroughly pulverized, and 4 quarts' soft water. The above must be thoroughly shaken when first put together. It is required to stand 24 hours, to dissolve the chloride of lime; then strain through a cotton cloth, after which add a tea- spoonful of acetic acid (NO. 8 commercial) to every ounce of the chloride of lime water. The eraser is used by reversing the pen holder in the hand, dipping the end of the pen holder in the fluid, and applying it, without rubbing, to the word, figure, or blot required to be erased. When the ink has disappeared. absorb the fluid with a blotter.—See SCIENTIFIC AMERICAN SUPPLEMENT, NO. 438, for information about gelatine copying pad or hektograph. (7)G. R. L. asks how to prepare a wash suitable for coloring an external wall a dark terra cotta tint. A. A wash for external work, said to be good, is formed in the following manner: Slake a shovelful of good lime in about a quart of warm blood, fresh from the slaughter house. Place in ordinary pail, and add a sufficient quantity of skim milk and beer grounds, boiled together, to fill the pail. Well stir the mixture, which will then be ready for use without the addition of water. and will stand the weather as well as oil paint. Another reported wash of excellence is formed by.mixing ol!,e gallon of lime slaked with one gallon of wood ashes, % pound of powdered alum or borax, and sufficient soft water to render the mixture of the consistency of cream. Color may be added to suit; 15 pounds of whiting and half a pound of fresh slaked lime, dissolved in skim milk, makes another hard and durable wash. To produce a terra cotta color, add 1 part of Indian red, 1 part of common lamp black, 3 parts of umber, and 1 to 2 parts of yellow ocher or chrome yellow, varying the quantity of the latter until the desired tint is obtained. (8)C. S. M. asks how to make an ink that will not appear on paper unless the paper is heated. A. Dissolve 1 fluid ounce common oil of vitriol in a pint of soft water. Stir well, and allow it to cool. Write with a clean pen. Whendry, it will be invisible; held to the flre, it turns black. (9)H. K. writes : A railroad train starting at the equator on a railroad running north, which rail wears the fasterast or west; and on which side would a train be most likely to run off therack? A. The east rail would have the greatest pressure, from the earth's motion, and if the train was running fast enough, it would be thrown off on east side. (10)Inquirer asks: 1. How can white country flannel shirts and drawers be washed without shrinking? Have hundreds to wash every two weeks, and the shrinkage soon renders the shirts too small for use. A. Care in rubbing and in the drying, after washing in tepid water, such as comes from experience, will make the shrinkage as little as possible, but the only sure way to insure such garments keeping their size is to dry them on forms, as do all the manufacturers of knit underwear. 2. What ingredients will form a wash to clean a brick church, now almost black, after 20 years' exposure in south side Pittsburg smoke f A. You will find the necessary information for cleaning brick walls in SCIENTIFIC AMERICAN SUPPLEMENT. No. 21. 3. Can you give a poor sufferer from asthmatic and bronchial ailments a remedy? A. There is a long and very explicit article on “ Bronchial Asthma” in SCIENTIFIC AMERICAN SUPPLEMENT, No. 171, by John C. Thorowgood, in which he gives several of his remedies. (11)Y. F. writes : If a steamer makes 8 miles per hour carrying 90 pounds of steam, with 150 pounds will she not increase her ppeed to 12 miles? A. If the boat has good lines for speed, possibly adding 60 per cent to the steam pressure, with capacity of supply for the 50 per cent increase in speed of engine, will give the boat a speed of 10 miles per hour. The slip of a wheel. paddle, or screw increases with the increase of speed. 2. What is the hottest steam used for driving an engine, and will steam, when too hot, become valueless? A. Steam has been used for power up to 50 and more pounds pressure. It becomes only valueless by burning packing and oil, The pressure may be carried up to a thousand pounds. (12)A. S. asks : 1. Is there any means by which to give very small wooden globules a permanent black or brown color, simply putting them in the solution ? A. Wash with a concentrated aqueous solution of extract of logwood several times; then with a solution of acetate of iron of 14° B, which is repeated.until a deep black is produced. 2. Where and for what price a square foot could Ibuy thin sheet lead to protect a table against acids? A. It is worth about ten cents per pound, and. can be procured from a dealer in chemical apparatus. (13)J. T. asks why the free silver on a silverprint is removed when immersed in a sQda bath (hyposulphite of soda), while that which is exposed to light is not. A. Sensitized paper is covered with albumen impregnated with chloride, and sometimes other haloidsalts of silver. By exposnre to the light these silver salts are reduced. Exposed under a negative, parts of the surface are protected from the action of light. When such a print, after exposure under a Ilfgative, is immersed in hyposulphite of soda, the silver chloride, etc., that light has not reduced is dissolved; the rest, by reduction, has been rendered insoluble in hyposulphite. and remains on the paper, constituting the print, which is ready for toning. There is no free silver in the print. The “ hypo.” removes the silver salts that have been unacted on by light. 2. Why do objects appear right side up to the senses, when they appear inverted on the retina of the eye? A. Presumably by experience and habit. Perception of distance is due to parallax, or distance apart of the eyes. 3. For solders and soldering see SUPPLEMENT, No. 20. (14)E. E. B. asks (1) how to make a solution for silver plating, to be applied with a sponge or flannel to brass or copper. A. You can make solution for silver plating on brass, etc., by dissolving 1 ounce of nitrate of silver in 1 quart of rain or distilled water, and a few crystals of hyposulphite of soda are added which .form a brown precipitate soluble in'a si ight excess of hyposulphite. Articles .may be silvered by dipping a sponge in the solution and rubbing it over the surface of the.article to be coated. 2. How to divide a circle into 860 parts. A. This. is generally accomplished by means of a protractor, costing from 25 cents upward, which can be procured from any dealer in mathematical or drawing instruments. (15)A. F.—The red coloring matter in thermometers is simply an aniline red dissolved in alcohol. (16)W. J. S. asks the composition and mode of manufacture of the so-called “ grease paints “ used by actors in making up. A. The principle is to make a dry powder somewhat darker than the desired tint, and then thoroughly mix this powder with some bland oil (as almond oil) or some fat (as perfumed benzoated lard) or some perfumed paraffinoid (as petrolatum), in the proportion necessary to produce the required color and consistency. (17)H. G.—Water will filter through a brick partition. See SCIENTIFIC AMERICAN SUPPLEMENT, No. .451, on Filtering Cisterns. (18)W. E. D. asks the process of casting bass relief tiles in bronze. A. The mould is made in sand from a pattern in the same manner as for ordinary brass work. For special description of bronze casting see SCIENTIFIC AMERICAN SUPPLEMENT, No. 101, and for finishing bronze work see SCIENTIFIC AMERICAN SUPPLEMENT, No. 39. (19)C. G. A.—The high polish on steel is produced by using Vienna lime on the buf. (20)J. W. C.-Riveted joints should always be calked. Tubes should be expanded to stop leaks. Iron borings sifted and l1Iade into a putty with Prince's metallic paint, white lead, and boiled linseed oil make a good joint for fianges. Joints that leak water will also leak steam, however small. (21)W. M. asks how to stain brass black; can it be done with fire or acids? It should be a dull black if possible. A. The best means for producing a black surface on brass or silver is said to be platinum bichloride, made by dissolving platinum in ni- trohydrochloric acid to saturation. Dip the polished work or rub the solution on with a small pad of cotton. After blacking, the abject is washed and lacquered. (22)W. H. B. asks how the roughness is made, like file cuts, as on the triggers of guns at the part where your thumb raises the trigger. A. The parts are iudented with a dull cold chisel in the manner of file cutting. The sharp edges are smoothed off in finishing. (23)E. A. Y. asks what is the cement used for putting on stained glass substitute. A. Nothing but the beet fish glue is used. (24)H. T. writes: I have a double convex crown object glass, 3 inches diameter, 36 inches focus. What kind, and what focus, should the eye glass be, so the object will be 'seen erect. and in a natural position ? A. For your eye glass use a concave lens of 3 inches or 4 inches negative focus. The 3 inch will give you a magnifying power of 12; the 4 inch a power of 9. making objects erect. (25)G. M. W. desires (1) a receipt ioji preventing rust on the spokes of a bicycle. A. Boiled linseed oil will keep polished metals from rusting »it is allowed to dry on them. 2. How to brighten (the nickel plating ? A. lIse a little rouge powder ap a chamois skin. (26)F. G. V.—Flowers may be preserved for many months by dipping them carefully, as soon as gathered, in perfectly limpid gum water; after allowing them to drain for two or three minutes, arrange them in a vase. The gum forms a complete coating on the stems and petals, and preserves their shape and color long after they have become dry. (27)G. I. asks : 1. What is ammonia used for in a nickel solution (double sulphate)? A. The double sulphate of nickel and ammonium has been found to be the best salt from which nickel can be deposited. The ammonia which it contains is held in chemical combination. 2. How to make oroide plating solution? A. This variety of gold is a mixture of several metals, and we know of no means by which it can be used as a solution to plate with. 8. How to make hydrous carbonate of copper ? A. By adding sodium carbonate in excess to a solution of copper sulphate. The resulting precipitate on being warmed assumes a green tint. 4. Is there any difference between hydrated and hydrous carbonate of copper? A."No. 5. A receipt for black color on bronze or brass? See answer to query 21. (28)G. R. S. desires a recipe for a safe effectual depilatory. A. The safest depilatory Is a strong solution of sulphide of barium made into a paste with powdered starch. It should be applied immediately after it is mixed, and allowed to remain there five or ten minutes. (29)J. B. H. asks the number of miles of railroad in the world. A. 294,071. 2. An easy way of preserving fiowers so they will retain their colors. A. See answer to query 26. (30)P. J. O'C. —Small cupolas have been made and used for iron castings, using 30 to 50 pounds to a melting. They require experience in their management, and are not economical. It is also more difficult to make the castings of even grade in small cupolas. (31)B. P. asks how to make porcelain glass or opal glass. A. Hot cast porcelain as manufactured in Pittsburg consists of: Silica6719 per cent. Cryolite" Zinc oxide 8'97 “ It is a milk white glass obtained by melting the above constituents together. (32)E. M. B. asks: What pressure per square inch will compressed air give, say of three atmospheres, and what is the ratio of increase? A. The atmospheric pressure Is 14'7 pounds. ElICh additional atmosphere adds the original pressure. thus 3 atmospheres is 44'1 pounds per square inch. (33)F. W. D. writes: I have frequently observed in the West Indies. a little after sunset, large bands of light emanating from the spot where the sun had just set, widening in approaching the zenth, thence narrowing to a focus on the eastern horizon, where they sometimes seemed to terminate in a mock sun. A. This phenomenon is common in more or less intensity in all parts of the.world. It is caused by clouds of various forms at or below the horizon intercepting the sunlight. The bands of light shining through broken clouds illuminate the air in streamers, sometimes reaching entirely across the sky, forming what appears to be diverging and converging rays in opposite horizons. These rays are really straight, and owe their apparent curved forms to the laws of perspective. (34)S. S. asks whether or not a boiler will evaporate more pounds of water per pound of fuel used when water is kept high than when kept low in boiler. A. A boiler with high water will lose more water by .vesicular admixture with the steam, or, in other words, will work wet steam. Low water makes dry steam unless the boiler is overworked. Dry steam makes its best work per pound of coal, and is accomplished at the low water line. This should always be a safe line., (35)M. P. P. asks a recipe for blacken ing the n. terior of telescope tubes—something thin, smooth, and dead black. A. Use lamp black or ivory black, rubbed up with 95 per cent alcohol. Then add a few drops of shellac varnish, just enough to make the lamp black adhere without gloss. Spread' quickly with a fiat camel's hair brush, or in small tubes with a feather or swab. (36)C. S. L.—Oil paintings that are freshly painted can be removed from the canvas by the application of a solvent, such as equal parts of alcohol and spirits of turpentine. If the paint is old and hard, the canvas can only be utilized.by covering the painting with several coats of white lead and Naples yellow. (37)F. C. C. asks about replating a revolver and erasing an engraved name therefrom. A. You can cut out the engraved space with a small chisel like a shallow mortise, and fit a piece of iron or brass in the space and tin it in. Then finish off the surface and replate the whole. If you undertake to fill up the space with tin or solder, it will not take the plating evenly, and will show the spot after plating. (38)G. F. K. writes: I am making a tool, part of which must be magnetic; what steel is best, also what temper is best, to magnetize same? A. Ordinary tool steel. Double shear steel is better. Harden at a cherry red, and draw to a straw color for magnets. Magnetize by contact with a strong magnet or electro magnet.