W. S. B. writes: In re “Musical Sands"-SUPlLEMENT, September 30th, p. 224-a good specimen exists at lanchester, Mass., where the sand of the beacb next the hotel creaks audibly when walked on. A. Very true ; and there used to be a sounding beach between Rockport and Gloucester, :Iass., when we used to summer on that coast. ''he waves raised the pebbles and smaller stones, and as they fell ba-ck they gave a musical tone by their rapid blows against each other. (12562) A. F. K. asks: 1. How much by weight and what number and kind (single or double covered) of primary copper wire is necessary for one induction coil such as is used for pump spark ignition on gasoline engines? A. 1"'01 the primary of an induction coil glvmg an inch spark use about 50 feet of No. 14, B.&S. single cotton-covered magnet wire. Double-covered wire may be used with some advantage in insulation, but a loss from the increased space taken up by the wire. 2. How much and what number and kind of secondary wire is necessary for the same coil? A. The secondary may contain 1” pounds of No. 36 wire. Silk-covered wire makes the smaller coil, but cotton-covered wire is stronger mechanically. 3. Is a condenser generally used with such coils? A. A condenser is necessary. You will find all details for making coils, machinery for winding, etc., described and figured in our Collins's “Making of Induction Coils,” price $3, a book everyone making a coil should have at hand. (12563) C. M. asks: In one of your older issues I saw how to make a luminous lamp. . I tried it, but it will not work. Heated pure olive oil for fifteen minutes in a small glass bottle, put in a piece of phosphorus and corked it up, but it gives off no light at all. The phosphorus sinks to the bottom of the glass. Can you tell me what is wrong? A. If you will take the cork out of the luminous bottle and shake the contents in the dark, so as to have some air 'from the outside euter the bottle, you will see the light, provided tue oil has dissolved any of the phosphorus. The oil will not shine unless some oxygen from the air can get at the phosphorus. We have had one of these for years which still shines whenever we shake it up and bring fresh air into contact with the phosphorus. (12564) J. T. P. asks: Can you tell if directions for making a pinhole camera have ever appeared in your publication? If so, in what number, and at what price can I get this copy? I wish to make an 8 x 10 camera. Have a book already telling how to make an apparatus to take 4 x 4 plates. This says make pinhole with No. 10 needle. Now tell me this: At what distance from pinhole should an 8 x 10 plate be, and what size needle should I make the pinhole with ? Also, what is the length of exposure for plates of this size? A. In the pinhole camera the size of the plate is not directly connected with the size of the hole at all. 'he distance of the plate from the hole for best definition is determined by the size of the hole. We give you a table from a French writer on this subject: Aperture. 0.012 inch. 0.016 inch. 0.02 inch. 0.024 inch. Distance of Plate. 4.33 inches. 7.87 inches. 11.9 inches. 17.32 inches. The pinhole should be perfectly round, and carefully beveled on both sides, as perfect as it can be made, if you would get best effect. It will include about 170 degrees. If then you secure a wide view at a distance of 8 inches with the aperture 0.016 inch, and wish a view giving a detail of some part of this wider “iew, you have only to move the plate back to 17.32 inches and use the 0,024 pinhole. 'he same width of view is taken in by the pinhole in both cases, but at the longer distance only a small portion of the view will fall on the same sized plate. The result is an enlarged view of a portion of the former picture. A wide range of the time of exposure is possible with skill in managing the development. With the smallest pinhole given, the exposure will be at least twenty-five times longer than with a lens giving the same size of picture, and with the longer distances the exposure must he greatly increased in length, probably two hundred times for the 17.32-inch distance. You will have to work these matters out for yourself by experiment. The light is very faint when spread over a large pia te, when thl' aperture is only 1/40 inch at the largest. 1911 Atlas S FREE With New To the readers of THE SCIENTIFIC AMERICAN who take advanta g e of this Offer now made in connection with Webster's NEW International XT .A New E d itio n O J. TVT C J.* n N ot AMereRevision B ut a INew iTeatloIl In Rich, Full Red Leather Binding. 490,090 Words and Phrases 6,000 Illustrations 3,000 Pages New Gazetteer of the World New Biographical Dictionary Readers of THE SCIENTIFIC AMERICAN can now secure this Newest an d Moat Complete Reference Libraryin Dictionary Form on the following remarkably easy terms: The entire work in {ull leather (with Atlas) Delivered for $1.00 and easy payment thereafter of only a f ew ce nts a week in the United States or C anada. WHAT EMINENT AUTHORITIES SAY United States Court of Claims: “We consider the work a distinct advance over other dictionaries which have come under our observation." Nicholas Murray Butler, President Columbia University: “Marks new advances even upon its excelknt predecessor." Dr. C. H. Parkfaurst, New York City: “it is a marvel of completeness and an indispensable feature of the library of every man who either reads or writes.1' New York Sun: “Not a word era definition in which some change for improvement has not been made." "To have this work in the home is like sending the whole family to college. “ To those who respond at once we will send a copy of an amusing “Test in Pro-nounciation” (with key), entitled “ The Americanization of Carver,” and also a “ Red Facsimile Booklet” of interesting questions, with references to their answers. The ATLAS is the New Census 1911 “ New Reference Atlas of the World.” containing 128 pages of maps beautiful printed in colors with marginal reference indexes. besides illustrated description of PAN AMA CANAL, all han dsom ely bound in red cloth. size 10^4 x 13% . M ail this coupon at o nce to G.&C. MERRIAM CO. SPRINGFIELD, MASS. Sale Publishers of GENUINE Webster Dictionaries For over 68 Years Home Office (Coupon) G.&C. MERRIAM CO., Springfield, Ma•• : Please send me. free of all obligauon of expense, a cnpy of .. Test in Pronunciation.” and also “Red Fac-slmile Booklet.” with special terms of your SCIENTIFIC AMEHlCAN free Atlas offer on the enlarged edition of . Webster's N EW International Dictionary." Name_............. The Scientific American C yclopedia ° Formulas Edited by ALBERT A. HOPKINS SIZE 6!x8% INCHES. I 077 PAGES. 200 ILLUSTRATIONS CLOT H, $5.00. HA L F MOROC CO, $6.50 POSTPAID ( This valuable work, which is partly based on the twenty-eighth edition of .. The Scientific American Cyclopedia of Receipts, Notes and Queries,” contains a collection of about 15,000 selected formulas, covering nearly every branch of the useful arts and industries. Never before has such a large collection of valuable formulas, useful to everyone, been offered to the public. ( This work may be regarded as the product of the studies and practical experience of the ablest chemists and workers in all parts of the world, the information given being of the highest value, condensed in concise form cODvenient for ready use. Almost every inquiry that can be thought of relating to formulas used in the various manufacturing industries, wjll here be found answered. ( The formulas are classified and arranged into chapters of related subjects, while a complete index, made by professional librarians, renders it easy to find any formula desired. ( Those engaged in any branch of industry will probably find in this volume much that is of practical use in their respective callings. Those in search .of salable articles, which can be manufactured on a small scale, will lnd hundreds of most excellent suggestions. It should lnd a place in every laboratory, factory and home. An eight page tescrPtive circular ant table of ils conlents will be fumisbet on application. 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