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 may prefer to address correspondents by mail. SPECIAL NOTE.—This column is designed for the general interest and instruction of our reader s,not for gratuitous replies to questions of a purely business or personal nature. We wilt publish such inquiries, however-whenpaid for as advertisemeta at $100 a line, under the head of Business and Personal. tTAll reference to backnumbers should be by volume and page. C. B., of Ohio.—The dampness of the wall of which you write us, will be hard to remedy entirely under the circumstances. A brick wall in contact withdamp shaded soil.will always be damp. You can help matters, however, by digging down below the brick on the outside and painting over the wall with pitch, such as is used on shipsbottoms. On the inside put up studs, and lath and plaster a wall, leaving a space be tween it and the brick. The studs may be of inch boards, as the only object is to get a wall not in contact with the damp brick. E. H. R., of 111.1—You are not the first who has considered the visformatrix, of a crystal the same, except in that of plants and animals-Profs Owen and Huxley entertain nearly the sameview.but when you ask what is the cause of this power, you go farther than those vigorous intellects deem it possible for philosophical inquiry to receive an answer. That is a subject for faith, not for physical research. W. A., of Ind.—The velocity of a 36-inch burr stone to do best work ought to be about 150 per minute. To grind ten bushels of corn per hour will require, according to Nicholsons Operative Mechanic, about three horse power. M. E. 0. C, of Wis.—The pressure in a boiler per square inch when it blows off, is found by multiplying the weight into the long arm of the safety valve lever, and dividing the product by the product of the short arm into the area of the valve in square inches, or a proportion may be used. In the special case you mention, where the long arm is 30 inches the short arm 3 inches, the area of the valve 5 square inches and the weight 125 lbs., the proportion would be 3:30 :: 125 :1250, the entire pressure sustained by the valve, which divided by 5 inches, the area of the valve, gives the pressure per square inch in the boiler. F. IT., of 111.— Will coal soot cause water to harden in a cis- tern ? Ans. In general it will not. If however, the soot precipitated on the roof comes from a zinc smelting furnace it might contain oxidea which would effect the water. The probable cause is the use of cement containing soluble compounds of lime. All new plastered cisterns render water more or less hard for a time. Q. J. B., of Vt.—Water engines are a very old device; you will find them described in Ewbanks Hydraulics, and In various other works.—Spencers Water meter is a small double cylind engine operated by water instead of steam, with slide valves and eccentrics. The objection to these machines arise from the inelasticity of water, and the liability of parts working under water to weart etc. H. K., of Mich.—Trv the alum and double ioi stopping holes in burr stones without the glue, the latter doefl harm rather than good. If the holes are large use some fragments of burr stone as part of the filling. E. W. L. C, of Ohio.—Shellac dissolved in alcohol is a good cement o make paper labels adhere to tin. The varnish should be tolerably thick. T. H. Gf., of N. Y. inquires, Can any of your numerous readers inform me, why it is, that, although many patents have been obtained for aerial machines, we hear nothing of their practical results ? The reason is simply that all aerial machines up to this time have been practically worthless. Jersey Farmer can obtain such information about sawmills as he wants, by putting an advertisement in the SCIENTIFIC AMERICAN. C. S. H., of Pa.—You ask What constitutes a days work for a draftsman? There is no rule in reference to it that we know of, but draftsmen in our office work about eight hours—That is as long as they ought to bend over the board. F. W. Woodward, of Winnsboro, S. C, states that there is an excellent quarry of oil andwhite stone near him and wishes to correspond with manufacturers. S. F. H., of Mass.,wisheS to know If the earth in proportion to its size is not as smooth and finely polished as a cambric needle ? We answer that in our judgment it is, but if our correspondent has any doubt about it we advise him to submit the question to an experimental test, and inform us of the result. It is a subject that deserves to be investi. gated. N. H. S., of N. Y.—Calcium was obtained by Matthiessen by the electrolytic decomposition of a mixture consisting of two equivalents of chloride of calcium and one equivalent of chloride of strontium. The mass may be fused in a Hessian crucible, in the center of which is placed a porous tube filled with the same mixture, and into this an iron wire passed through the stem of a tobacco pipe is inserted. This wire is connected with the platinode of a battery, the zincode of which consists of a plate of iron bent into a cylindrical form, and immersed in the melted mass exterior to the porous tube. The calcium is reduced and preserved from oxidation by so regulating the heat that a film of solidified salt shall form upon the surface of the mixture in the porous cell. Lies Bodart obtained it more easily by fusing iodide of calcium with an equivalent quantity of sodium. See Millers Inorganic Chemistry, page 407. S. U. B., of Mich.—There is no difficulty in superheating steam in pipes to 300 Fah., but it is doubtful if the temperature of a room for kiln drying can be kept, by that means, to that temperature. Much of the heat is lost by radiation. Direct heat from a properly constructed furnace is better than steam heat for kiln drying purposes. N. F. P., of N. J.—We do not hold ourselves responsible for the statements of advertisers in our columns, under whatever head they may choose to address our readers. The Business and Personal, is an advertising column; we do not feel at liberty to express an opinion as to the value of the devices therein mentioned, or on the character of the advertisers. Our opinion of the Whitlock Exposition is freely expressed on page 280, current volume, in an editorial article. C. W. I., of Iowa, asksii there is any practical rule for the position of a water wheel in a draft tube; whether there is any point in such tube at which awheel will give a greater percentage of power than at any other. We hardly know what this correspondent means. He may suppose that more force may be gained by conducting the power (head of water) through a tube, to the wheel, at a distance from the source, than in receiving it direct from the source, or fall; but it is evident that the closer the wheel to the force—the less friction and consequent waste—the more power will be delivered. J. E. C, of Mass.—Malleable cast iron is simply ordinary cast iron subjected to a red heat for hours, or days, according to the size of the articles, they being packed in iron scales or pulverized specular iron ore, the object being to combine the oxygen of the oxide with the carbon of the iron. A visit to any malleable iron concern will show the modus op-erandi better than we can describe it in a column. Under this heading we shall publish weekly notes oj some of the more prominent home and foreign patents.