CORRESPONDENTS who expect to reCeive answer. to their letters must in all case8, sign their names. We have a right to know tho8e who seek'in ..?o:er:s????1;??d??f8 bbe:i:?il?8 8ometime8 happen8. we may prefer to ad SPECIAL NOTE.-This column is designed for the general interest and in.?? ?n??iV;??O.?l??verti8emet8 at $1'00 a line, under the head of ".BU.Si: All reference to back numbers .hould be by volume and page. M. E., of Conn.—The grand discovery of atmospheric pressure was made only a little more than two hundred years ago—in 1643—by Tor-ricelli, a pupil of Galileo. Torricelli's announcement that the air had weight, that its pressure sustained the mercurial column, and that every body went about, bearing upon their persons an air burden of fifteen tuns, was received with scorn and ridicule by many of the scientific men of his day. BntFascal, a French savant, acknowledged its correctness "If," said he, "it be really the weight of the atmosphere under which we live that supports the column of mercury in Torricelll's tube, we shall find, by transporting this tube to a loftier point in the atmosphere, that in proportion as we leave below more and more of the air, there will be a less column of mercury sustained in the tube." He carried the tube to the peak of Puy-de-Dome, a lofty mountain in Central France, and found that the mereury gradually fell in the tube in proportion as he ascended. In1646, at Eouen,Pascal showed that a column ofwater, 34 feet high, was sustained by atmospheric pressure. 30 inches of mercury, and 34 feet of water have nearly thesame weight. T. H. J., of Texas.—The best lubricant for iron spur gearing, is undoubtedly good sperm oil. The following proportions for teeth may be considered good practice: Depth of pitch line = three tenths of the pitch. Working depth of tooth sixth tenths of the pitch. Bottom clearance one tenth. Whole depth to the root of the tooth seven tenths. Thickness of tooth fi.ve elevenths of the pitch width of space six elevenths. Other proportions are however used. B. W. S. C., of Ind.—You can run a steam saw mill successfully from a counter shaft under your saw mill, said counter shaft to be driven from a pulley 7 feet in diameter on a main shaft thirty feet distant but your belts will need to be well proportioned. In short you will need a man of experience and skill to adjust everything so as to insure success. J. N., of Ohio.—You will find in our advertising' columns, advertisements of safety valves and ofher steam apparatus. It will be evident to you upon a moment's reflection. tlwt it is not our place, even were we disposed, to recommend any of these to the exclusion of others Ifyou need counselable engineers, who also atlvcrtise with U3, will give it to you. 397 w. N. G.,of N. Y.—Both the plumber and yourself (are partly wrong. N.o water can be foreed into a range boiler, or any othet boiler so long as the pressure of steam In the boiler is greater than the pressure of the water. The pressure of water pourea In through a funnel put Into either the hot or the cold water pipe, near the top of your boiler would, notbe enough to force it in against a slight pressure of steam. The funnel used for fllUng the boiler during the low water, should have been placed in the cold water pipe, and the faucet of the hot water pipe should have been opened while the filling was going on, to relieve the boiler from steam pressure. W. H. C., of 111.—The rule you refer to for computing the horse power of steam engines is correct. Your engine would, according to your statement, have 19'04 horse power. We recommend you to get " Bacon's Revision of Porter's Work on the Steam Indicator," noticed in another column, and u Auchincloss' Link and Valve Motions." Both works arc published by D. Van Nostrand,2SMurray and 27 Warren streets, New York. C. M. T., of Mass.—The suggestions you make are valuable but as we have recently published an illustrated description of a safety heating apparatus for railroad cars (see page 40. current volume) together with similar suggestions, we respectfuHy decline your communication. W. H. E., ofPa.—The soap test for water referred to is a tincture ofsoap, made by dissolving fine soap in 75 parts of water by weight and then adding an equal volume of rectified alcohol. The amount of the tincturewhich makes a permanent lather when padded to water is an indication of the hardness or softness of the water. J. S. E., of Mass.—The furrow in the tough sod after the severe storm of which you write us, is evidently the work of lightning. Such occurrences are not unfrequent, and the peculiar hissing sound of lightning you mention has also been often observed. T. E. T., of Ga.—A compass needle is deflected by beds of iron ore but such a needle would not assist you in the search for gold or sU-ver The divining rods of which you speak are in our opinion humbugs. J. M. D., of 1ll.—To specify all the uses of peroxide of manganese wouldoccupy much space. Both it and phosphate of iron are used in medicine. P. H. D.—It is a well known property ofloaf sugar that it becomes luminous when rubbed with a hard substance in the dark. The subject has been frequently referred to in this column. C. H. C, of 1ll.—There is no doubt in the minds of scientific men of the existence of atmospheric tides, caused by the attraction of the moon in the same way that ocean tides are produced. S. M. A., of Conn.—You will find an account of ancient tools in u Appleton's American Cyclopedia,'* with references to authors upon the subject in arUcle " Copper." J. C., of Ill.—Your communication d ;es not suit us. It is offensively personal. J. A. L., of Ohio.—You may learn to translate the French language into English by the aid ofbooks, but to speak it you will need the aid of a living teacher.
This article was originally published with the title "Answers to Correspondents" in Scientific American 21, 25, 396-397 (December 1869)