HINTS TO CORRESPONDENTS Names and Address must accompany all letters, or no attention will be paid thereto. This is for our information and for publication. References to former articles or answers should give date or paper and page or number of question. Inquiries not answered in reasonable time should be repeated; correspondents will bear in mind that some answers require not a little research, and, though we endeavor to reply to all either by letter or in this department. each must take his turn. Buyers wishing to purchase any article not advertised in our columns will be furnished with addresses of houses manufacturing or carrying the same. Special Written Information on matters of personal rather than general interest cannot be expected without remuneration. Scientific American Supplement referred to may be had at the office. Price 10 cents each. Books referred to promptly supplied on receipt of price. Minerals sent for examination should be distinctly marked or labeled. (1) A, .H...M. asks: In ad ynamo not using batteries or permanent magnets. where is current of electricity denerated that makes the field magnets electro magnets? A. In. the armature-, and thence part or all the 'current is 'carried around the field magnets. (2) L. J.. asks- about cleaning petroleum pipes.. If a powerful magnet be fixed in the scraper, would a compass passed on the outside not indicate the scraper's position? Is the pipe stoneware or iron? A. The 'pipe is iron. A magnet in the scraper might to a slight extent deflect or affect a poised iron or steel unmagnetized needle. The compass would be useless, as the metal of . the pipe would affect it. Two difficulties in connection with any such plan are :1. The metal pipe would, by distributing the magnetism, act as a magnetic shield, so that it is questionable if any evidence of its presence could be obtained. 2. A strong magnet, byattraction to the sides of the pipe, would help to obstruct the movement. . (3) J. S. M. asks for description of . the Jamin magnet, its form and construction, and the quality of steel' used. A. Jamin's horseshoe magnets are made ofplates of steel magnetized separately and arranged one within the.bend pf the other, or concentrically.. For tempering, etc., consult Supplement, Nos. 302 and 318. (4)-D. . B” of Ohio.—A State court has no authority tQ. prohibit the use of a patent pending the action in a suit brought thereon, which necessarily involves thequestion of infringement. (5) O. J. S. . asKs (1) if coal gas decreased in bulk one-half .will develop double the pressure. A; It' will.. a. Row .many.cubic feet of coal gas would be contained- in a reservoir 15 inches diameter and 3 feet in length, at a pressure of 200 pounds per square inch? A. 49-2 cubic feet. '