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 ana instruction ofour 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 "Busk ness and Personal." WWAll reference to back numbers should be by volume andpage. A. H. C, of—wishes to know the amount of bituminous coal usually consumed in heating one tun of nail plate, we suppose he means during the cutting process; can any of our correspondents give the information ? J. E, B., of Mass.—Congress adopted the meter as a standard of measurement, July 27,1866. D. B., of Ca.—The substance used for gumming stamps is gum dextrine. It is applied like other similar substances. L. VrB., of N. C.—The best thing to remove rust from needles is the common method of scouring them, by sticking them repeatedly in a small bag of fine emery. G. B. F., of M. T.-We were well aware of the fact that String-fellow exhibited a small engine at the Aeronautical Society's Exhibition in London, as we noticed it in the SCIENTIFIC AMERICAN, at the time, but no such engines can be obtained at the present time. The journal to which you refer is in the habit of taking our replies to correspondents and publishing them as its own. E. R., of N. C.—There is no machinery in use operating upon the clock principle, that is capable of driving a watchmakers lathe. A small, cheap, and efficient power is much wanted for light work.