CORRESPONDENTS who expect to receive answers to their letters must, til all cases, sign their names. We have a right to know those who seek information from us; beside, as sometimes happens, we may prefer to address correspondents by mail. ' SPECIAL JVOTE. This column is designed for the general interest and in struction of our readers, not for gratuitous replies to questions of a purely business or personal nature, we will publish such inquiries, however, when paid for as advertisemets at $1*00 a line, under the head of "Business and Personal." WAll reference to back numbers should be by volume and vage. C. L. H., of Ohio. An aqueous solution of gum-arabic is the best varnish for leaves and flowers. W. S. S., of N. Y. Your communication upon rat-proof buildings fails to explain how they should be constructed. In its present shape we cannot regard it in any other light than as an advertisement of a pat ent. E. J. F. of Me. An application of glycerin to the tubs will not injure the taste of butter, and the article is harmless. You can get it at the druggists. L. O. B., of Ind., wishes to know a practical method of scouring wool oil containing petroleum, out of cloth or yarn. He says the yarn when this oil has been used, turns yellow after standing awhile, and never comes out as white as when pure lard oil has been used, and when he attempted to scour with lye or country soap, he could notget good results. Can any of our correspondents give the desired information. Wm. S. C, of--------The usual estimate of a horse power, 83,000 lbs. raised one foot in one minute, is the work that average horses will perform steadily with suitable machinery. The best method of applying the power of a horse to propulsion of machinery is in our opinion, the endless chain norse power in common use if properly made and set with reference to the machinery tobe driven. J. Van O., of Pa. We have practiced the following method for drying chlorine gas, with excellent results. Take of pumice stone a quantity of small fragments the size of a pea, soak them in strong sulphuric acid, then calcine them until acid fumes cease to De disengaged. These fragments are then re-saturated with sulphuric acid and inclosed in a tube through which the gas is passed in the ordinary manner of drying other gases. The sulphuric acid will seize the water contained in the gas the latter passing over in a dry state. J. E. C, of Iowa. When the same length of belt is to be used to ive diiferent speeds, the centers of the pulleys remaining equi-distant, the diameter of the driver must be increased as that of the driven is diminished, or vice versa and the speed of the circumference of both the driver and driven pulley will increase exactly as the diameter of the driver is increased. The number of revolutions made by the driven pulley will be to the number of revolutions made by the driver, as the diameter of the driven pulley is to that of the driver. Thus if the diameter of the driver be 4 and that of the driven 2, and the number of revolutions of the driver be 60, the proportion will be, 2: 4 :: 60: J.20 the number of reyo* lutions made by the driven pulley. F. P. H., of Mass. We know of no " water-proof glue " for uniting wood. Many recipes are -published which assume to be waterproof, but we do not believe in any of them, as glues are dissolved in wa -ter, and of course water will re-dissolve them. India rubber (virgin) dissolved, 4 parts in 30 parts naphtha, or benzine, and 65 parts ground or po w * dered shellac melted in it make as near an approach to water-proof glue as anything we know. It will also unite metal and wood if the surfaces are clean. Molesworth, in his " Engineer's Pocket Book " gives the fol lowing: " For a glue to resist moisture, melt 1 lb of glue in two quarts o skimmed milk. A strong glue, add powdered chalk to common glue. His marine glue is similar to that, the formula of which is given above. We cannot tell you where " machines for plaiting silk fishing lines " are to be obtained. J, S. C, of Pa. We do not consider the question of the precise instant when the gun receives the recoil of the explosion whether at the time of ignition of the powder, or when the bullet leaves the bar" rel, thus creating a vacuum of sufficient value to occupy a space" in our columns. H. A. S., of Me., says he saw in the Scientific American about two years ago a statement of the erection of a flour mill in New York, to hull the wheat before grinding. He asks "What became of it and why don't the owners advertise ?" S. W. H., and Bro., of Mo., say that they use an exhaust pipe of tin, four inches diameter, for leading their exhaust to a heater. It drops two feet from the engine cylinder, traverses ten feet horizontally, and then rises four feet to the heater. In starting the engine March 5th, the horizontal portion collapsed. ' What" they ask " is the reason?" The only cause is the pressure of the atmosphere without), and a vacuum within the pipe. Probably an examination would show that the communication with the atmosphere was closed either by the action of the back pressure valve opening outward'or by the water. Sheet tin is in any case poor material for conducting steam. W. S. T., of K H Number of feet traversed by minute of your little engine is 562; pressure, about 4 lbs on piston, result less than one-sixteenth of one horse power. T. F. H., of Conn. A good dark bronze dip is made by dissolving iron scales (scales from the forge) lib., arsenic 1 ozM zinc 1 oz. in 1 Ib., muriatic acid ; the zinc tobeaddedto the solution just before using. The metal to receive it should be cleaned by diluted acid. L. V. G., of Ohio. For an ordinary foot lathe for wood or light metal work, a wheel of iron from 30 to 36 inches diameter is suffl cient for a driver, weighing 150 to 175 lbs. The live spindle should run in brass composition or Babbitt metal.