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 This is for our information and not for publication.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. Inquiries 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 must take his turn. Buyers wishing to purchase any article not adver tised in our columns will be furnished with addresses of bouses manufacturing or carrying the same. Special Written Information on matters of personal rather than general interest cannot be expected without remuneration. Scientifio American Supplements referred to may be had at the otBce. Price 10 cents each. Books referred to promptly supplied on receipt of price. Minerals sent for examination should be distinct)) marked or labeled. Our attention has been called to the statement made in a recent reply to a query that the Wehnelt interrupter was not patented. It seems that a patent is held upon this de-rice by a well-known firm of makers of surgi cal instruments in New York, of which we were not informed at the time of our writing. (10631) W. D. S. asks: I have some thing that I am unable to solve, and write you for information. If you like you may publish the discovery. However, I hold the right of discovering for myself and friends. I inclose you a common white feather, which you will hold close to one eye, and close the other eye. Hold your hand between you and the light about two feet from the feather, and you can see the bones as if looking through an X-ray. Y'ou can also see through solid substance, such as a silver dollar, etc. A. The interesting experiment you send us has been made a great many times. It is an effect of the interference of the waves of light when they pass the edges of narrow slits or openings, such as those between the barbs and barbules of a feather. A fine piece of cloth, such as bolting cloth or fine silk, will also show the snmc phenomenon. That you do not "see the bones, as if looking with an X-ray," will be evident, if you look at a rod of wood or iron through the feather. You will see a bone in the wood or iron, just as you do in the finger. Thls curious effect is produced by the bending of the rays of light as they come from the rod of wood or the finger, and then pass through the narrow spaces of the feather or cloth. After they are thus bent they cross each other, and seem to bring light to the eye from botk edges of the wood or finger. There is there fore a border or margin on both sides of the finger or rod of wood, from which some light seems to come to the eye. The central dark strip is seen in its real iilumination, less than the edges show, and this you call the bone. (10632) A. F. M. writes: 1. We have here some very favorable sites for storing water for power and irrigation, and desert lands and mines which would be benefited. Cement, how ever, is too expensive, having to be carried by pack animals across 80 miles of bad country. It is a country of granite, with abundant tim ber for constructing large cranes. We propose to hoist la rye granite blocks 4 to 5 feet cubic into the dam-site. These would not be dressed blocks, and the spaces between would be filled with smaller blocks. The blocks would be got by blasting. The wall would be built 140 feet wide at the base, and about 100 feet in depth ; inclose explanatory sketch. 2. We want to carry the water in a flume made from the coarse local pine. The temperature varies from freez ing in winter to 100 deg. in summer ; and heavy thunderstorms are followed by hot sun, which will cause the boards to warp and split. We propose to nail the boards tightly in place, and line them with asphalt paper to prevent leaking. The water will be free from foreign matter, which might wear the paper. 3. Would a dam stand, if constructed of a wooden wall lined on the inside with 8-ounce canvas well oiled with linseed oil and Venetian red or other metallic paint? The dam not to be more than 20 or 30 feet high and its base to rest on low cement dams, well cemented to solid rock foundation, so that neither the wood nor its cloth covering comes in contact with the soil, but only with water or air. A. In reply (1) we would consider that the dam you describe is practical and safe if the stone Is well laid and bonded, and the spaces between the larger stones properly wedged with smaller ones so as to make the whole one solid mass of stone. We would advise concrete facing thicker than you suggest—about C feet thick at the bottom, and 2 feet at the top. A good water-tight joint must be made between the concrete and the foundation bedrock and care must be taken that the body of the dam rests properly on a bedrock foundation which has been cut in steps to receive it. (2) In regard to the flume we think the best plan for keep ing same water-tight is to line it with burlap laid in hot pitch tar or asphaltum and again coated with the same material after it has been laid. Care should be taken to lap joints well. Ci) Your timber dam is practical up to 20 feet or so. We give sketch of same with dimensions figured for safe loads up to 20 feet. To keep the planking water-tight coat with asphaltum, or with burlap and asphaltum put on as de scribed for the flume in case the cracks be tween the planks are wide. If the water is always kept in storage the timber will not rot where it can always be kept wet. It is better to keep the air away from the bottom of the posts for that reason. (10633) F. McC. writes that a re cently installed steam-heating system does not work satisfactorily. He asks how the draft may be increased, and if a forced draft could be installed if necessary. A. You can deter mine whether insi ffh'ient draft is the cause of your heating boilers failing to give satis faction by having the flue gases and ash residues analyzed, to show whether combustion is sufficiently complete. You can readily in crease the draft artificially by inserting a blower at any point in the flue or chimney thus : The blower being driven by a small electric motor or otherwise as convenient. The.B. F. Sturtevant Company, 114 Liberty Street, New Y'ork, would give you full particulars. We are pleased to advise you, but are not the people who Installed the heating plant responsible? You should be able to compel them to put the plant in such condition as to give satisfactory service without further cost to you. (10634) U. J. W. asks: Will you kind ly give me the necessary information to make a spark coil such as is usually used on motor cycles, the three terminals for three cells of dry batteries? A. You will find in RITPI.K-MKNT No. 1281. price 10 cents, an article giv ing the Information you wish regarding the Ignition of a gas engine. The data for a, 367 sparking coil are given in that article. ? coil with a single winding is used, with an iron core. The self-induction of the discharge gives the hot spark for ignition. (10635) ?. M Jr., says: In your issue of September 28, under the heading No. 10G1S, you publish a solution of the problem of making a concrete gasoline tank tight. Your idea is to saturate the dry walls and roof with melted paraffine. May I call your attention to the fact that paraffine is extremely soluble in gasoline, and that while this treatment might serve excellently to make the tank re tain water, it would be worse than useless in the case of gasoline. From my experience with concrete, I should think a coating of cement plaster—1 part cement to 1 part sand—applied thoroughly to the bottom and sides of the tank, would materially help to tighten it. This plaster should be not less than % inch thick at any spot. In case this failed a coating of hot tar might be applied through out, provided the parts to which it is applied are thoroughly dry. The tar should be very hot, and most carefully applied. Damp ness in the walls will make it peel. When one considers how difficult it is to make a sim ple concrete structure even water-tight, the problem of retaining so volatile a substance as gasoline in concrete offers difficulties that make one skeptical of its success. And it is certainly against reason to suppose that such can be achieved through the use of any sub stance soluble in gasoline. (10636) F. A. McD. asks for formulas for plate and film developers, using the follow ing active substances: (1) Pyrogallic acid, (2) hydroquinone, (3) eikonogen, (4) metol. 1.— Pyro Solution. Distilled or pure ice water. 6 ounces Oxalic acid ............. 10 grains Sodium sulphite (crystals). 1 drachm Pyrogallic acid........... 1 ounce Alkali Solution. Water................. 64 ounces Sodium carbonate (crys tals) ................ 2% ounces Sodium sulphite (crystals) 3 ounces To prepare the developer, add 2% drachms of the pyro solution to 8 ounces of the alkali solution. In case of warm weather or over exposure, add 10 to 40 minims of a bromide of potash solution made up of 1 ounce of potassium dissolved in 10 ounces of water. 2.— Hydroquinone ............ 2 grains Sodium sulphite (crystals).. 60 grains Water .................. 1 ounce Alkaline Solution. Sodium carbonate ........ 60 grains Water................... 1 ounce To one ounce of the alkaline solution add two ounces of the hydroquinone solution and one ounce of water. If the density is insuf ficient increase the hydroquinone. 3.— No. 1. Water ................... 40 ounces Sodium sulphite .......... 2 ounces Eikonogen ............... 1 ounce Alkali Solution (No. 2). Water ................... 3 ounces Potassium carbonate ...... 1 ounce Take 2 ounces of No. 1 and from 1 to 2 drachms of No. 2. Add more of No. 2 if de velopment proceeds too slowly. 4.— No. 1. Metol ................. % ounce Water ................. 32 ounces Dissolve the metol in 16 ounces of water, then in the second 10 ounces dissolve 5 ounces of sodium sulphite (crystals) ; add the two solutions, which forms the stock solution. No. 2. Water ................. 32 ounces Potassium carbonate. ... 2% ounces No. 3. Bromide of potassium solution, 20 grains dissolved in 32 ounces of water. For average exposure take 2 ounces of No. 1, % ounce of No. 2, and add 3 ounces of water. If development proceeds too fast add % drachm of No. 3 solution. For under-exposure add more of No. 2. If % ounce of hydro quinone is added to the No. 1 metol solution a developer of considerable power is com pounded. Ortol Developer.—A vigorous developer giv ing a brownish deposit ; keeps well in two so lutions. No. 1. Water................... 10 ounces Meta bisulphite of potassium. 40 grains Ortol ................... 80 grains No. 2. Water .................. 10 ounces Sodium sulphite (crystals) 1 ounce Potassium carbonate...... 160 grains For correct exposures add 1 ounce of No. 1 to 1 ounce of NO. 2 and 1 ounce of water. If the exposure is unknown add 1/10 part of No. 2 to No. 1 and 1 ounce of water, then add a little of No. 2 at a time until develop ment proceeds moderately. I'yro Acetone Developer.—Acetone is substi tuted for the usual alkali. The following is used by Lumi re Brothers : Water .................. 100 parts Sodium sulphite (anhy drous) ................ 5 parts Acetone solution ......... 10 parts Pyrogallic acid ........... 1 part.