CORRESPONDENTS who expect to receive answers to their letters must, m allcases,sign their names. We have a right to know those who seek information from ua; beside, as sometimes happens, we may prefer to address correspondents by mail. SPECIAL NOTE.—This column is designed forthegeneralinterestand instruction of our readers,not for gratuitous replies to questions of a purely business or personal nature. We wUl 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 beby volume and page. T. P., of Mo.—It you wish to prevent the splitting of your hollow cutters or reamers in hardening, drill a small hole from the side to meet the large hole at its apex; [otherwise there is no chance for the steam to escape and cracking is sure to result. W. H. B., of Pa.—You can clean a clogged file or rotary cutter effectually by smearing it with oil lightly, exposing it to a forge flame a moment, and then carding it with the card that every machinist or filer uses. You may heat it so hot you cannot bear your hand upon it, but the temper will not be drawn. S. R, of Me.—Ordinary rosin powdered and melted on the plate by a slight heat will give a temporary resistance to acids. In fact any resinous substance will form a base for a resistant to acids. T. L., of Pa., wants to use an incombustible spongy coherent substance like platinum sponge but cheap. He says however that it need not possess all the properties of platinum sponge; all that is required is incombustibility, sponginess, and coherency. H. N. C, of Pa.—A stream of cold air directed forcibly against a heated bar of iron keeps it hot, by the combustion of the iron itself. The oxygen of the air is brought so closely in contact with the heated metal, and is supplied in such quantity that it unites with the iron producing all the phenomena of combustion, among which is the evolution of heat. A. W. P. of Pa.—There is no difficulty in making a speaking tube convey audible sounds one hundred yards. Biot, a celebrated French philosopher was able to hear low whispers through a tube of iron 3,130 feet in length. The straighter it is the better. We dont think putting it beneath the ground essential. Put it where it will be most convenient, J. W. H., of Pa.—Self-sealing cans might be made that would perhaps allow the air to escape when the cans are heated, and close themselves hermetically upon the contraction of the air in cooling, but we are not aware that any such are used. TheexpulBion of the;airis the only object to b e attainesn heating the fruit, where the modern process of canning is used, and if that is not thoroughly done the heating is detrimental rather than beneficial. J. M., of 111.—A good varnish for maps or water color paintings, is made of genuine Canada balsam and rectified oil of turpentine, equal parts. Mix; set the bottle containing the mixture in warm water, and agitate until the solution is perfect; then set in a warm place a week to settle, when pour off the clear varnish for use. J. M. H., of Seminole Nation.—There is special legislation for the Indian reservations which can be found on consulting Brightleys Digest. Should the infringement of a patent be committed therein by a res ident of some other State or Territory, a suit would be good, could the party be served, or the infringing machine be found within the jurisdiction of the court in which the suit is commenced. The Territories have District Courts which are severally invested with same jurisdiction as the U. S. Circuit and District Courts of the U. S.