Correspondent's 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, a s sometimes happens. we may pre ferto address correspondents by mail. SPECIAL NOTE This column to designed for the general interest and in- er business or personal nature. We will publish suchinquiriess, horn eve; when paid for as advertisemets at $1'00 a line, under ihe head of “Business and Personal All reference to backnumbers should be byvolume andpage. L. W., of Mo.You may determine the variation of the magnetic needle by the following formula : To the log. secant oftlie latitude (rejecting the index), add. thelog. sine of the sun's declination, corrected for time and place of observation; the sum will he tile log. sine of the true amplitude, to be reckoned from tlie east in the morning or the west t endtg th nation. Then if the trueand magnetic amplitudes he both north or both south, their difference is the variation, but if one be north and the other south, their sum is the variation. To determine whether the variation be to the west or east, suppose yourself looking toward the point oftlie compass representing the magnetic amplitude. Then, if the true amplitude be to the right of the magnetic amplitude, the variation is east, and vice vena. Youmayalso determine it by first reducing the sun's decimation to tlie time and place of observation and computing the true altitude of the sun's center. Second, substract the sun's declination from 90 degrees when the latitude and declination are of the same name, or add it to 90 degrees when they are of contrarynames. The remainder,orthe sum, will b e the sun's polar distance. Third, add together the sun's polar distance, the latitude of the place, and the altitude of the sun. Find and reserve the difference between half this sum and the sun's polar distance, and to the log. co-sine of the difference thus found, add the sum of tlie log. secant of the altitude, the log. secant of the latitude, and the log. co-sine of the half sum ot the sun's polar distance, the latitude, and tl'e sun's altitude (rejecting indices). Half the sum of these logarithms will be the sine oi half the sun'strueazimuth. Then if tlie true and observeclaziniuths 'b bothon the east, or both on the west side of the meridian, their difference is the variation, but if on opposite sides of the meridian their sum is the variation. J. S. G. Jr., of D. C.The black sand you sond contains iron ore to which your magnetized lmnife blade adheres. Its presence is not a. special indication of valuable' ores. The earths you speak of are species of clays, and the colors are due to the presence of various mineral substances in small quantities. On the line oftlie Baltimore and Washington Railroad there are fine'examples oftliese colored earths; also on the Philadelphia&Baltimore roads. The earth that you speak of as hardening by exposure to the atmosphere is found in various places. J. G. P., of Pa.In a drying loft heated bv. a system of steam pipes on the floor, and in which a constant circulation of air is to be maintained, the air should be admitted through registers in the floor. In your case parallel openings, one inch in width, in the floor between the parallel pipes, would be best, with ventilators at the to, to render tie circulation as diffused as possible. These ventilators had better be numerous, small and well distributed than few and large. L. H. W., of D. C.The amount of borax put into water may be largely in excess of that needed for laundry purposes. Its alkaline rc- action is too feeble injure the fiber of linen, cotton, or woolen textures . We 61.o not now recollect any book containing the precise information you seek in your other queries. You had better address a lot ler of inquiry to Henry Carey Baird, Industrial Publisher, Philadelphia, Pa. G. C. B., of Conn.To take plaster of Paris casts of medals, wood-cuts, etc.,you must oil the object,the impression of which you wish to obtain. Then place it, face upward, in a box or other appliance, to keep the plaster from running off while soft. Next mix rapidly the plaster with water to the thickness of cream, pour it on the mold and. let it harden, which it will do in a short time. W. B., of N. Y.Mere motion in air has tho effect to heat it rather than to cool it. It is only when the motion mixes colder air with heated air that the motion has anything to do with the cooling. The heat- ingand cooling of air are subject to the same laws as govern the heating and cooling of all other fluids. G. S., of 111.We have tested the alcohol and camphor barometer pretty thoroughly this summer, and have found it wholly unre- reliable. 'Ye have not much faith in any barometer as a mere weather ; indicator. R. M. A., of Mass.You need not fear any ill effects in your boiler, piping, or engine, from the blasting powder detritus, left in the well you are digging, after one or two thorough rinsings. How to Get Patents Extended. Patents can be extended, for seven years,under the general law, but t is requisite that the petition for extension should be filed with the Commissioner of Patents, at least ninety days before the date on which the patent expires. Manyflf,atents are now allowed to expire which could be made profitable under an extended term. Applications for extensions can only be made by the patentee, or, in the event of his death, by Ins legal rep resentative. Parties interested in patents about to expire, can obtain all necessary instructions how to proceed, free of charge, bywriting to MUNN&CO., 37 Park Row, New York. W. P. A., of Mass.The information you seek cannot be given in the space we can allot you. Dussauce's “ Treatise on Tanning,” published hyHenry Carey Baird, of Philadelphia, Pa., contains it in full. P. J., of N. Y.Pure gold is very malleable and in that sense plastic. When properly manipulated almost any form can be given to it without the use of heat or the aid of any solvent. S. 'V. P., of N. Y.The hair dressing of which you inquire is made of alcohol one pint, pure glycerine two ounces, and pure water one half pint. Scented to suit taste. © 1869 SCIENTIFIC AMERICAN, INC. OCTOBER 2, 1869. Indorsement of” the Great Western Improved Advertising Company. The following is one of many eimilarindorsements received by this Company: PBitfCtPAL OFFICE WILSON's S. 11 Co., j CLEVELAND, OHIO, August30,1869. Sitwell Harris, Esq., Manager Great Western Improved Newspaper Advertising Co., St. Louis. Mo.: SIR :In compliance with your request, I cheerfully certify, that during the past year and a half,we have done a large amount of advertising through your agency, and our contracts, embracing over two hundred papers, have been carried out to our entire satisfaction. Yours truly, W. G. WILSON, President Wilson S. M. Co.