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 address correspondents by mail. SPECIAL NOTE.-This column il designed for the general interest and in struction of our reader stnot forgratuitous replies to questions of a purely business 00ftotfwrI. will publish such, inquiries, however* when paid for as advertisemets at $1'00 a line, under the head of “Business and Personal All reference to back numbers should be by volume and page. P. M. M., of N. Y., writes for an explanation of a singular case of collapse occurring in a low-wines still. The cause of this collapse must have been the removal of internal pressure by condensation, the external pressure of the atmosphere then acting to crush in the crown of the still. How this could have occurred with the worm open and the stream of high wines passing through being only a small fraction of its capacity, while the ordinary operation of distillation was in progress, seems unaccountable. It is probable 80me condition has been overlooked by our correspondent. S. R. of Vt.—We have often used both a rotating slide valve and oscillating cylindrical valve in hydraulic machines, and either will work well for a time. The cylindrical valve is however better replaced by a tapering one like those used in ordinary water cocks, as that will enable you to take up all wear on the valve and seat. This wear soon makes a cylindrical valve leak where much work is required. The rotating slide valve, however, will keep tight a long while with fair treatment. D. V., of Va.—A machine is only a means of transmitting and applying force to work. It of itself does nothing. When moved by tha application of force, it does not even transmit the whole of that force but absorbs a portion of it. How, then, can you by placing a machine between force and work, expect to apply more force to work with, than without the intervening machine. A seeond look at your computation will show you your error. C.E., of Mich.—What is called puddled steel is made by stopping the process ofpuddling; iron at the precise time when sufficient carbon remains in it to form steel. When the puddling is carried beyond this point, more of the carbon is combined with oxygen, and passing off in the form of carbonic acid, leaves the reduced metal in a state called malleable iron, the principal difference between which and steel is the less amount of carbon it contains. D.M. T., of N. Y.—Two bodies moving in contact with each other, and at a common velocity, can neither of them take motion from the other. There are, however, some very nice points connected with this subject which we cannot discuss here ; but you may safely conclude that when one body is imparting motion to another body, the latter must be moving with less velocity in some direction than the former. D. K. M., of Pa.—You can prevent in a great degree the rusting of an iron vessel in which water is boiled by greasing the interior and allowing it to, as the housekeepers say, “burn on.” Wipe it out with a greasy rag and then let it heat till it smokes freely. Repeat this several times and you will not be troubled again soon. R. M., of Ill.—To compute the length of an arc of any number of degrees, radius being given, multiply the radius by 710 divided by 118. Multiply this product by the number of degrees in the are, and divide by 360, and you will have the length of thearc in the denomination by which theof the radius is expressed. L. T. D., of Md.—Where a force pump is -employed to force water to a great hight, it is the best practice, in our opinion, to use more than one check valve, as the valves only add to the power required to work the pump by their weight, while the wear will be distributed provided the valves are properly adjusted. M. M. G., of N. Y.—You can coat malleable iron castings permanently with copper by the use of the electroplating process, which you will find fully described in the “ Practical Metal Workers' Assistant,” published by Henry Carey Baird, of Philadelphia. That._work also gives the other information you desire. C.J. H., of Pa.—To kill knots before painting apply a paste of wet lime to them. When the paste dries apply a hot iron to the knot which will melt out the pitch, and the lime will absorb it. The spots may be rubbed down smooth and then paint applied. L. C. D., of Wis.—The details of the art of encaustic painting, as practiced by the ancients, arc not now known. According to Pliny, it is probable that the vehicle of the colors was melted wax, but attempts at imitating this method in modern times have been unsuccessful. D.S., of N. J.—Glue is not soluble in oil, as you might easily have determined by an experiment. It may therefore be used to coat over the insides of oil casks, and will in great measure prevent loss from leakage. C. B. H., of N. Y.—Agates, carnelians, and other hard stones, are sawed with small metal plates armed with diamond dust. You can get such work done by Michael Fox&Co., No. 1 Maiden Lane, New York. E.C. H., of R. I.—Your device is as old as Vitruviu8, who describes it exactly. Bell-shaped reflectors of sound were used in the Corinthian theaters, and were introduced into Rome after the taking of Corinth. T. P., of N. C.—The aluminum bronze is the strongest alloy yet discovered. Its composition is ninety parts of copper to ten of aluminum. C. L. M., of Ca,—What have been called Egyptian pebbles are a species of agate or jasper. G. W. B., of Wis.—The stone you send appears to be agate.