CORRESPONDENTS who expect to receive answers to their letters must, m allcasea,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 in-struction of our readers,notf or gratuitous replies to questions of a purely business or personal nature. We will publish such inquiries, however, when paid for as advertisemets at $100 a line, under the head of "Bust-ness and Personal." fW"AH reference to back numbers should be by volume andpage. H. H. K., of Mass.—To extinguish a kerosene lamp safely turn the wick down until the flame is low and blow under the glass. If turned down until almost outflowing in the top of the chimney is said not to be dangerous. Horse powers'are so common and of so many different forms that we could hardly designate any particular one. None, however, is more generally and acceptably used than that where the animal travels an inclined, endless track, too common to require more than a mention. N. and H., of Conn., desire to know the address of the maker of the annular (diamond) drill such as is now used in drilling at Hell Gate. We saw it at work in Titusville, Pa., boring for oil through solid rock, in 1865, but do not know the name of the maker. Some of our readers may answer this question. T. K., of Mich., propounds the following : Suppose a pound weight will just bring down a spiral spring six inches in hight, how many pounds will be required to bring down eight spiral springseach six inches in hight, each spring being separated from the other, not counting anything for the weight of the springs, and they being placed one upon the other?" The separation of the springs and the placing of one upon the other seem to present a contradiction. But, in any view, a satisfactory reply to the question can be obtained only by experiment. The laws governing the action of springs are not sufficiently defined to construct a reliable theory. If the end sought is expected to be of practical benefit tha simple experiment is warranted. R. G., of Mich., and T. B., of Pa.—The reply to "M. S. W." page 55 current Vol., is erroneous in so far as the square inches were not taken into consideration. The question, as understood, was the relative downward pressure of columns of water of varying diameters. It is evident that if the relative pressure to the square inch is considered, the aggregate amount of pressure coincides with the number of square inches exposed. W. G. W., of Ala.—The reaction of water in an Archimedes screw, will cause it to rotate if left to itself and a supply be given to it at the top. We do not think the combination you suggest, would be valuable. J. O. B., of Ala.—We do not know where air-guns can be purchased in this city. You coul*d probably find out by advertising in our paper. , r K J., of Me.-i ead melts & |G deg. Fah., but melted ad may attain a much higher temperature. H. G., of Fla.—All that is necessary to convert oyster shells into excellent lime is to burn them, that is to heat them until the carbonate of lime which constitutes their bulk is decomposed and the carbonic acid gas expelled. A line or two in our paper under the head of "Business and Personal " will probably put you in communication with the parties interested in the sale of patent lime kilns. L. W., of Me.—The problem you send us is either wrongly enunciated, or involves an absurdity. A body may revolve around another body, or two bodies may revolve around a fixed or moving point in space, but it is absurd to speak about two bodies revolving at the same time around each other. If you mean that two bodies are revolving around axommon moving center, the data are still defective. Theirpaths cannot be determined without the radii of revolution as well as their velocities. J. G. S., of Pa.—The less the pressure under which gas is discharged through a meter, the more cubic feet will be required for the same amount of light, all other things being equal. Partially stopping of the inlet pipe, as it reduces the pressure, is therefore only putting money in the pockets of the gas companies. This fact is well known and has often been made the subject of accurate experiment. In fact it has been made the subject of legislation in foreign countries, and we think in some of the United States, gas companies being required by law to deliver their gas under a given pressure. IN". B., of N. Y.—A very good cement may "be [made "by melting together in an iron dish, equal parts of gutta percha and common pitch, and when wellstirred pouring the mass into cold water. This becomes perfectly fluid at a temperature of 100 degrees. It is a useful cement for many purposes. A still stronger cement, for leather and cloth, is made of 1 lb. gutta-percha, 4 oz. india-rubber, 2 oz. pitch, 1 oz. shellac, and 2 oz. of fine sperm oil. The ingredients are to be melted and thoroughly stirred together. The cement is to be used hot. G. H. D., of N. Y.—The smooth side of the belt is the best to put next the pulley. C. D., of Ohio.—There is no patent on the toy steam engine, about which you inquire. W. M., of Pa.—The water on an overshot wheel, should be applied as near to the summit of the wheel as possible. The processesfor making self-raising flour are*usuallypatented. S. A. O., of N. Y.—It is said that soaking plaster casts in a thin and warm solution of glue will, after they become dry, render them quite hard and tough. Silicate of soda or better the double silicate of soda and potassa, will also give them greater consistency. J. O. S., of N. Y.—The consideration of the size of driving wheels is not alone sufficient to determine the tractive power of an engine. Large driving wheels are used when great speed is dssired, abd when a proper adjustment of other parts is made theyanswerthe purpose designed. W. Y., of N. Y.—A gold colored lacquer is made by taking seed lac 3 ozs, tumeric 1 oz., dragons blood U oz., and digesting them for a week in one pint of alcohol, shaking often and finally pouring off and straining the solution. S. K., of Mass.—To dress a solid rubber and emery wheel when it gets out of true or becomes, glazed, heat a block of iron of considerable size quite hot, and place it near the edge of the wheel while it is 125 inmotion. Atthe same time using an old file or its equivalent as a turning tool you can by working upon the opposite side soon true up your wheel and removfl the glaze. M. S. Gt., of N. Y., asks how to " figure out the gears on a lathe to cut two, three or four threaded screw and how to measure or count them." He has#found out from the SCIENTIFIC AMERICAN how to arrange for single threaded screws. There is no difference between the arrangement of gears for cutting double, triple, or quadruple threads and that for single threads. The face plate of the lathe must be divided into as many equal parts as there are to be threads, and the tool set out by eachmark in succession. In counting multi-threaded screws there is no difficulty. The number of the threads maybe seen by looking at their terminals on the end of a screw, and then, laying the rule on the screw, counting one for each of the series; Thus a three threaded screw should be counted one for every three threads, etc.