CORRESPONDENTS who expect to receive answers to their letters must, m all cases, sign their names. We have a right to know those who seek in formation from, us; beside, as sometimes happens, we may prefer to ad dress correspondents by mail. SPECIAL NOTE.—This column is designed for the general interest and in struction of our readers,not for gratuitous replies to questions ofapurely business or personal nature. We will publish such inquiries, however when paid for as advertisemets at $l'0l) a line, under the head of "Busi ness and Personal." fWAll reference to back numbers should be by volume andpage. A. S. G., of D. C.—The power necessary to drive a train of wheelwork seven hours,so that a driven wheel,one inch in diameter,may revolve 40 revolutions per minute.with four pounds at its periphery, may be computed as follows: 1 inchM3*1416=3*1416 inches, 3'1416 inches X40—125* 664 inches the circumferential motion per minute. This multiplied by 420 , the number of minutes in seven hours,—52778*88 inches=4398*24 feet. As four pounds of resistance are to be overcome through this distancein sevenhours wehave for the power required. 17592*96 foot-pounds. Toac-complish this work by a weight falling through a space of seven feet, th weight must weigh one seventh of 17592*96 pounds.which is 2513*28 pounds making no allowance for friction, which will, we estimate, require in your case, twentyper cent morepowerthan this, making the entire weight required, nearly 3,016 pounds. C. R. F., of N. J.—As good a tool as you can use for roughing down a large wooden drum onthe shaft where it is to run, is an old file ground down to a sharp point. This will not split out fragments even tliough it should catch in a knot or a nail. When the approximate form has been attained you can use a gouge, chisel, and sandpaper to finish. A rest good enough for the purpose can generally be ma.de of hard-wood plank suspended withnailsirom and braced to the joists overhead. T. R. J., of Mass.—The best tool to burr off small castings is a vulcanized emery wheel. If youhavemuchsuchwork todo it will pay foritselfsooninthesaving of files. To remove therust from such castings, put them—a bushel at once—in a tumbling barrel, with leather cuttings and chips, they will soon wear bright. This will not however take the rustfrom the inside of small hollow castings. To clean such, dip in dilute sulphuric acid—1 part of commercial acid to ten of water—wash in hot lime water, and dry in the tumbler with dry sawdust. H. H., of Ohio.—Experiment can only determine your first querry. We think, however, that you will find itdifflcult to make an alloy of platina and silver, whose fusing point will be exactly what you require. The asbestos used in making clothing is a variety of amphibole not containing much alumina. C. R, of Vt.—Saws may be made to cut so smoothly that a very good finish may be obtained by sand-paperina* only. You will find such saws at work in manufactories of veneers, and it would pay you be fore proceeding further with your invention to visit some such establishment. S. McN., of Cal.—The substance you send us is nothing but wood charcoal mixed with a little sand and sufficient plastic clay to ce ment it into lumps. How it came fifty feet below the surface where you f ouni it must be a matter of conjecture. Charcoal is however, unchangeable at ordinary temperatures, and it may have remained there a thons and years. A. G., of N. Y.—Good strong glue is the best thing for fixing emery to cloth belts for polishing wood. P.O., of Mich.—You are mistaken i n supposing a cylindrical adjutage will permit the largest flow of water. The form which permits the greatest flow is that of a truncated cone with its base in the direction of the flow, F. J. E., of Md.—There is no danger of poisoning wells by using sulphate of iron (green vitriol) as a disinfectant. It is often administered as a tonic by physicians. C. R. M., of Del.—We presume you could find a market in New York for blackberry wine. It has quite a reputation as a i medy in diarrhea, dysentery, and similar complaints. S. R. M., of N. Y.—A foot-pound is the measure of the force necessary to raise one pound one foot high. - Gr. S., of 111.—Your weather indicator will no doubt operate well, but a more simple barometer may be made by simply placing inside a glass tube, or long vial, a dime's worth of pulverized gum camphor, filling with water, and then hermetically sealing the tube. C. S., of Pa.—Your article on " Property in Invention," is so like a great number of others received on the same subject that we cannot for the present give it insertion. J, W. R., of N. J.—Glass is probably the best article you can use for breaking the electric circuit.