CORRESPONDENTS who expect to receive answers to their letters must,'//, all cases, sign their names. We have a right to know thorn 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, 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 ''Bum-ness and Personal." %WAll reference to back numbers should be by volume andpage. 0. I. C, of Ind.—The best and cheapest material for making-concrete pavement with gravel is coal tar from the gas works. J. K., of Boston.—A person has no right to construct a velocipede or any other machine for his own use, which would infringe on an existing patent. J. W. E,., of N. J.—A permanent magnet will gradually lose its power when the armature or keeper is removed from the poles. The circumstance you mention will not affect the action of this law. C. Y., of N. Y.—You can easily make your name upon steel by the process called etching. Coat over the tools with a thin layer of wax or hard tallow, by first warming the steel and rubbing on the wax; warm until it flows, and then let it cool. When hard, mark your name through the wax with a graver and apply aquafortis (nitric acid); after a few moments, wash off the acid thoroughly with water, warm the metal enough to melt the wax and wipe it off with a soft rag. The letters will be found etched into the steel. W. R. J., of Pa.—A perpetual motion as the term is understood in mechanics, is a machine that creates the force by which it is driven independently of any external cause. It must of course be able to startitself and remain inmotion until its parts are worn out. Any machine that depends for its motive power upon any force derived from any external source as heat from coal, electricity from the corrosion of metals by chemical reagents, etc., is not a " perpetual motion." A body im mersed in a fluid, subjected to pressure would require more power to move it than when the pressure is removed. W. G., of N. Y.—A hollow tube in order to possess maximum strength must have its external and internal diameters in the proportion of 10 to 7, The external di.imeterof a bar being 5 inches, its internal diameter should be 7-10 of 5 equal to 3-5 inches. "When this proportion is maintained the hollow bar has twice the strength of a solid one containing the same amount of material. The absolute strength of beams, geometrically similar in form, is as the squares of their corresponding dimensions. These data will enable you to solve the particular case you mention. F. D., of La., wishes to know the composition and mode of manufacture of gold colored and violet colored inks. We have seen these and other colors lately displayed in store windows, and we are inclined to to the opinionthat coal tar, or aniline colors are the coloring bases. We do not understand the manufacture. Perhaps some correspondent can give the information. J. E., of Mass.—We were correct in our reply to B. M. R., ot Va., that plumbago is a compound of carbon and iron. The purest ever yet discovered contains 98-55 per cent of carbon and about 1 per cent of iron, the remainder being made up of othc-r impurities. Graphite or plum bago is formed or produced artifi cially in the slags ol furnaces in the pro cess of reducing iron ores. To your second question we rely that, when dissolved, common salt is muriate of soda, when dry it is chloride of sodi um. This may seem paradoxical, but if we had room we could give you an explanation of its reactions under treatment which would be convincing. J. R., of Ohio.—"Gas lime" is regarded as being a good and cheap fertilizer upon soils deficient in lime. J. F., of Ind., asks how to temper blacksmith's anvils'. We never imagined there was any peculiar difficulty in it. We have knon of a " new laid," or new faced anvil, hardened simply by heating to the prop er degree then immersed in a tank of cold water, face up, so that two or three inches of water were above the face, and a constant stream of co d water from a hose pipe kept playing over the face