CORRESPONDENTS who expect to receive ansioers to their letters must, in all cases, sl'jm their names. We have a right to Tcnoiv those who seek in-formation frormts; heside, as sometimes happens, we may prefer to ad dress correspondents by mail. SPECIAL NOTE.—This column iS designed for the general interest and in struction of our readers, not fw gratuitous regies 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 '?'Business and Personal." All reference to back numbers should be by volume andpage. J. C. C, of Miss.—The smell in the collected rainwater standing exposed to the air in an open wooden vessel does not probably ariae from the wooden vessel in wMch it stands, but from the accumulation of organic matter in it. You can purify it by leaching it througJi charcoal dust, which, placed in a cask, will make a good filter. When the charcoal loses its deodorizing power it can be renewed by heating it in a closed vessel. E. A. D., of Va.—The resisting power of a non-conductor is not diminished by its relative position in regard to otter conductors. So if a non-conducting substance be placed between heated gases and the walls confining them, it will not cease its action though another better conductor should afterward be placed between it and the hot gases. J. C. K., of Iowa.—While we have no doubt that a locomotive with six-feet driving wheels, having a train attached, may have, at timea, ? attained a speed of a mile per minute on down grades, we do not believe it ever drew atrainatthatrate. The highest speed at present attained upon any railway, is from London to Liverpool, where trains run at the rate of 50 miles per hour. C. A. P., of 111 —To make tragacanth mucilage take of traga-canth, a troy ounce, and boiling water a pint. Macerate with occasional stirring 24 hours. Then rub the mucilage together thoroughly to produce uniformity, and strain forcibly through linen. Add creasote until the odor is faintly perceptible in order to prevent mold and decomposition. If you wish to make a thick mucilage for sealing enveloves, etc., it will be sufficient to put some lumps of the gum in a small bottle and put in cold water. Let it stand until it softens. If too thick add water, if too thin add gum. L. L. VanD., of Neb.—The pressure of a liquid on any portion of a lateral wall is equal to the weight of a column of liquid which has for its base this portion of the wall, and for its hight the vertical distance from its superficial center to the surface of the liquid. A column of water 144 feet high weighs 63J lbs. for each square inch of base, if of uniform size throughout. A. A. C, of Mich.—The moisture which accumulates upon the outside of a pitcher containing cold water is condensed moisture from" the air. The temperature at which water thus deposits upon cold sur faces is called the dew-point, and is higher or lower according to the amount of moisture held in suspension in the atmosphere.