Hints to Correspondents N ames and Add ress 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 This is for our information and not for publication.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 o a a r Inquiries not answered in reasonable time should d 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 wis hing to purchase any article not advertised in our columns will be furnishedwith addressesof houses manufacturing or carrying the same. 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. Books referred to promptly supplied on receipt of price. lUi neralsent for examination should be distinctly marked or labeled. (6671) W. B. MCP. asks: 1. Is there any foundation for the theory of getting better health by sleeping with the head to the north ? If so, why ? A. It is doubtful if any particular benefit is derived from sleeping with the head to the north. Yet it has been asserted by nervous people that a difference was noticeable in their temper and composure with changes of sleeping position in regard to the magnetic polarity of the earth. 2. Where can I get the glass tubes, retorts and other implements necessary for a few simple experiments in chemistry? Is there any firm that manufactures them that issues a catalogue ? A. Address Eimer & Amend, Third Avenue and Eighteenth Street, New York, for catalogue of chemical furniture and supplies. (6672) W. F. O. writes : When steam boilers are full of water, is it possible to raise the temperature and pressure to a. dangerous degree ? If not, the water jackets surrounding gas engine cylinders might be filled in a similar way and the necessity of maintaining a continuous circulation of cold water avoided. A. There is danger in heating a closed boiler full of water. The expansion of the water would rupture the boiler if there were no safety valve. It is a common practice to use an iron open tank filled with water and connected with the water jacket of a gas or gasoline engine in such way that a continuous circulation of water through the cylinder jacket takes place, the large surface of the iron tank being sufficient for keeping the water cool. (6673) J. W. i ys : I want to know what is the best way to keep the windows in a store from sweating and spoiling the goods. A. To keep frost. etc., off plate glass windows, keep the inside air dry, or inner sash tight, so that the air m window inclosure will be cold, and ventilated from the outside. A partial remedy is to have ventilating openings in the top of the window casing. A thin coat of pure glycerine applied to both sides of the glass will prevent any moisture forming thereon, and will stay until it collects so much dust that it cannot be seen through. Surveyors can use it to advantage on their instruments in foggy weather. In fact, it can be used anywhere to prevent moisture from forming on anything, and locomotive engineers will find it particularly useful in preventing the accumulation of steam as well as frost on their windows during the cold weather. (6674) N. B . W, asks: 1. What i S the best proportion of air and gasoline vapor for a gasoline engine? A. 25 to 40 volumes, according to the composition of the gasoline. 2. At what temperature will it explode? A. At a full red heat, say 220 Fah. 3. Describe Teslas electric motor. A. See our SUPPLEMENT, Nos. 692, 944, 1025. (6675) H. A. W. asks how to make French mustard. A. The following is M. LenormaDds recipe: Flour of mustard, 2lb.; fresh parsley, chervil, celery and tarragon, of each ).2 oz ; garlic, 1 clove (oi head); 12 salt anchovies (all well chopped); grind wel together, add salt, 1 oZ.; grape juice or sugar to sweeten and sufficient water to form the mass into a thin paste by trituration in a mortar. When put into pots a red hot iron is momentarily thrust into the contents of each, and i little winevinegar added. (6676) H. J. T. asks how to make gelatine capsules. A. Dissolve in a water bath 10 parts oJ gelatine, 2).2 parts of sugar, 14 parts of gum arabic in 1( parte of water. Take iron pins, the lower ends of whicl are pear-shaped and slightly oiled, dip in this solution whei it is lukewarm. When the gelatine films are congealed detach them, and place in holes of the same size in wooden forms, to dry. The capsules are filled with th desired medicine and closed with a drop of the same so lution.