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Kindly keep your Queries on separate sheets of paper when corrcsponding about such matters as patents, subscriptions, books, etc. This will greatly facilitate answering your questions, as in many· cases they have to be referred to experts. The full name and address should be given on every sheet. No attention will be paid to unsigned queries. Full hints to correspondents are printed from time to time and will be mailed on request. (12501) F. M. E. asks: Is it worth while for some of our aviators to watch for an opportunity and rise in the wake of a rain cloud and see the rainbow a complete circle? It would certainly be a booutiful spectacle and will be seen by very few. A. Whether one would think it worth while to rise In the wake of a rain cloud on the chance of seeing a complete rainbow, we cannot say. It would be a unique experience, if it should be successfully accomplished. (12502) J. W. C. asks: 1. A cubic foot of air in a globe or vessel that weighs, say 2 pounds, would require a certain weight to sink it in water. Now if it Is compressed to 64 cubic inches, and the container is same weight, would its buoyancy be the same, or would more or less weight be required to sink it in water to same depth 7 A. A cubic foot of air at ordinary density weighs about an ounce and a quarter. If it Is in a globe which weighs two pounds, the whole will weigh the Rum of two pounds. If the globe has a volume of one cubic foot, it will displace a cubic foot of water, which weighs if fresh about 62% pounds, and it will be buoyed up by a force of about 60 pounds 6 % ounces, the dilerence of these two weights. Now, if the air be compressed to 64 cubic inches, it and its container will weigh the same as before, but will be buoyed up by the weight of 64 cubic inches of water, which you can calculate from the weight of a cubic foot as given above. You will find that the globe will just float in fresh water, the buoyant f(rce beiug only a very few ounces. 2. What would be the pressure per square inch if a cubic foot of air is compressed to 64 cubic inches ? A. If a cubic foot of air is compressed to 64 cubic inches, the pressure at the same temperature will be twenty·seven times the normal pressure, which is 405 pounds per square Inch. 3. Have you any books or papers that would be helpful to an amateur in building small rowboats and motorboats? If so, please give me price of same, or tell me where I can get such books. A. We can supply the following bMks on boat building: “Proctieal Boat Building for Amateurs,” by Neison, $1; “Canoe and Boat Building fOl Amateurs,” by Stephens, $2; “Nine Motor Boats and How to Build Them,” $1; “How to Build a Motor Lauuch from Plans,” by Davis, $1.50; “How to Build a Motor Launch,” by Mower, $1. Upon receipt of the prices quoted above, we will forward these books postpaid to your address. (12503) G. K. asks: My friend's boy, a youth of 18 years, came to me and asked me if iron made into powder form could be electrified, either with positive or negative electricity. Further, he wished to know what the result would be, if both powders were blown through a pipe, each one separate at the point of contact. A. Powdered iron can be electrified if it is insulated, and If it were blown from a pipe while charged with either positive or negative electricity, the particles of iron would repel each otler and be driven apart in all directions. If there were two iron powders, one electrified positively and the other negatively, of equal quantity, upon mixing the two powders they would discharge each other and there would be no charge of electricity left, since Iron is a conductor of electricity, and can retain it only when it is insulated. We hope that this explains what your young friend wished to know. It would be profitable for such a young man to have the Scientific Am e rican for himself. We hope he may become a subscriber to It. (12504) A. C. M. asks: Will you please be so kind as to give me a formula In your queries column for making a nickel· plating solution for electroplating? A. For a small bath for nickel·plating, dissolve 12 to 14 ounces of the double sulphate of nickel and ammonium, per gallon of water, in a stone jar which is perfectly clean. Add water till the solution is of a density of 1.03 to 1.06, or from 6.5 deg. B. to 7 deg. B . . For large baths a very different method is taken to shorten the time. You will find full instructions in Langbein's “Flectro·Deposition of Metals,” $4, “1 in Watt's “Electro·plating,” $4.50. A very good sIllall book of very practical instructions is Van' Horne's “Modern Electroplating,” $1. We shall be pleased to supply any of our readers with these books upon receipt of price. (12505) W. S. K. asks: Can you furnish me the address of a concern where I can buy a metrescope? It ,is used in enlarging of pictures, etc. I have never seen one, and can only give you an idea of it. It is made of glass. A. We do not know any instrument which is kl'lPt for Rale to be used in enlarging pictures. This is usually done by dividing the picture into squares, and the paper upon which it is to be copied into the same number of squares. Then, as you will see, any portion of the picture will be drawn to fill the square on the copy which corresponds to the square which it fills in the original picture. A plate of glass ruled in squares laid over the picture would be very convenient but you can rule fine pencil marks over a picture and afterward erase them, or you can make a light frame and draw a thread across it to form the squares. Other ways will sugglst themselves to you for making the square, cheaper than buying a frame. (12506) D. S. asks: 1. Supposing that a retort or any other receptacle was filled with pure air (with of course provision for Its expansion) and heat was applied to the outside of the retort sufficient to raise the temperature of the air in the retort to say 1,000 deg . or 1,200 deg., would that air be fit to breathe after it was cooled to the ordinary atmospheric temperature? A. If pure air containing only oxygen, nitrogen and the other gases which are found in minute quantities in pure air should be inclosed in a retort and hooted to 1,500 deg, Fahr., and again cooled, no change would tal
