W.S.W. asks: How is the best rosin, used on violin bows, prepared?--W.F. asks: What is a simple method for washing clay for brick and tile making?--E.S.D. asks: What is the best kind of wood to construct a guitar? * * * * * COMMUNICATIONS RECEIVED. The Editor of the SCIENTIFIC AMERICAN acknowledges, with much pleasure, the receipt of original papers and contributions upon the following subjects: On Rheumatism. By A.R.E. On Postage Stamps. By E.B. On Boiler Explosions. By G.B.B. On Reaching the North Pole. By J.H.S. On Heating Street Cars. By P.T. On a Hybrid Fruit, By R.S.B. On an Air Vessel. By J.T.R. Also inquiries and answers from the following: E.B.M.--F.F.F.--N.B.H.--B.B.--O.F.--R.V.J.--F.M.--N.B.C.--C.F.E.--W.T. --C.W.C.--T.F.--C.A.S.--S.N.M.--J.R.D.--P.J.D.S. * * * * * HINTS TO CORRESPONDENTS. Correspondents whose inquiries fail to appear should repeat them. If not then published, they may conclude that, for good reasons, the Editor declines them. The address of the writer should always be given. Inquiries relating to patents, or to the patentability of inventions, assignments, etc., will not be published here. All such questions, when initials only are given, are thrown into the waste basket, as it would fill half of our paper to print them all; but we generally take pleasure in answering briefly by mail, if the writer's address is given. Hundreds of inquiries analogous to the following are sent: "Who sells a tool for truing up a crosshead wrist? Who sells tools for refitting steam valves without unscrewing them from the pipes? Who sells spoke-turning lathes? Who makes machinery for freeing wool of burrs and dirt? Where can tungsten, or tungsten steel, be procured, and at what price? Who sells silicate of alumina and silicate of potash?" All such personal inquiries are printed, as will be observed, in the column of "Business and Personal," which is specially set apart for that purpose, subject to the charge mentioned at the head of that column. Almost any desired information can in this way be expeditiously obtained. * * * * * OFFICIAL. INDEX OF INVENTIONS FOR WHICH LETTERS PATENT OF THE UNITED STATES WERE GRANTED IN THE WEEK ENDING January 18, 1877, AND EACH BEARING THAT DATE. [Those marked (r) are reissued patents.] * * * * * A complete copy of any patent in the annexed list, including both the specifications and drawings, will be furnished from this office for one dollar. In ordering, please state the number and date of the patent desired, and remit to Munn & Co., 37 Park Row, New York city. * * * * *