CORRESPONDENTS who expect to receive answers to their letters must, in all cases, sign their names. We have a right to know those who seek information from us; beside, as sometimes happens, we niay prefer to address correspondents by mail. SPECIAL NO TE This column is designed for the general interest and in -8truction of our readers, not for gratuitous replies to questions of a purely business or personal nature. We will publish such inquiries hoioevei-when paid for as advertisemets at $1*00 a line, under the head of " Business and Personal." IW All reference to back numbers should be by volume andpa.ge. S. S. GK, of Mass. We know of no recipe for preventing damp woods from splitting when exposed to heat. Such a discovery would be valuable. J. M. B.. of Mass. The most fusible alloy with which we are acquainted is made of 8 parts of lead, 15 parts bismuth,4 of tin, and 3 of cadmium. It is Called " Woods metal," and is wo think patented. It melts at 140 degrees Fah.andhas a specific gravity of 9*4. F. GL D., of 111. Two theories of the origin of tho earth's magnetism have prevailed. The older, that of Hansteen, conceives the earth to be possessed of independent magnetism havingits f ocusnear the earth's center. It is now claimed that the ;crust of the earth and not its interior is the seat of terrestrial magnetism. To account for the pointing of the magnetic needle to the north, would be to assign a cause for the attraction, a positive pole for the negative pole of a magnet. This has never been determined. p. r(j of----------If you will refer to page 20, Vol. XIX, Scientific American, you will find your question in relation to apparent variation between position of crank and piston of an engine fully answered, and illustrated by a diagram. J. P., of Ontario. Securing belt splices by shoe pegs is not objectionable when rivets are not at hand; we have frequently practiced it with as good results as when sewed with lace leather. In " butting " o r m eeting b elts the crossings o f the lacings should b e o n the outside o f the belt; the straight stretches on the inside next the pulley face. W. H. P., of N. Y. Case hardening to be quickly performed is done by the use of prussiate of potash. This is powdered and spread upon the surface of the piece of iron to be hardened, after the iron is heated to a bright red. It almost instantly fluxes or flows over the surface, and when the iron is cooled to a dull red it is plunged into cold wa -ter. Some prefer a mixture of prussiate of potash 3 parts, sal ammoniac 1 part; or prussiate 1 part, sal ammoniac 2 parts, and finely powdered bono dust (unburned) 2 parts. The application is the same in each case. Proper case hardening, when a deep coating of steel is desired, is done by packing: the article to be hardened in an iron box with horn, hoof,bone dust, shreds of leather or raw hide, or either of these, and heated to a red heat, for from one to three hours, then plunged in water. D. S., of Minn. Common yellow brass for turning may be made of copper 2 zinc 1. For heavy work, tin, copper, and zinc are used in the proportions of tin 15, copper 100, and zinc 15, or tinl3, copper 112 zinc 1. J. GL S., of Va. The magnetic meridian does not correspond with the geographical meridian, except in very few places. It also is subject to variations. The magnetic needle is also subject to so many variations that an attempt to establish the true meridian by its use, would cause you considerable trouble. You can get it neru- enough for your pur pose, by allowing the sun to shine through a vertical slit at noon when' the sun is neither fast nor slow of clock, provided you can take time from a clock which is right with the sun or varies from it by a Known rate. C* you may get it quite accurately by describing a circle on a level surfte and placing a vertical wire, seven or eight inches long, in the cento Through the top of the wire should be drilled $ Email hole to permit U sun to shine through. The beam of light passing through the hole will cross the circle once before noon and once in the afternoon. Watch when it crosses the circle in the morning, and mark the point of intersection. Repeat the operation in the afternoon. The points of intersection will lie at equal distances from the true meridian. Join the two points by a line, and bisect it to find its middle point. A line joining this middle point and the center of the vertical pin will lie on the meridian. It is better to draw several concentric circles and perform the same operation with each to secure accuracy. They should be so drawn that the beam will cross them between the hours of 9 and 12 in the morning. The best time to do this is about the summer solstice. It will be snfflciently accurate for your purpose; however, to do it now. Glass lamp chimneys should, be annealed at the time they are manufactured. We do not tiling you will succeed in annealing them in a stove oven. E. P., of Ind. We believe there are a number of makers and dealers in india-rubber tires for velocipedes, but we cannot remember their address. Better advertise for what you wish in our “ Business and Personal” column.
This article was originally published with the title "Correspondence"