(22) P. O. asks: If I admit steam 100 lbs. pressure in a cylinder 15 x 24 inches, and cut the steam off when piston has traveled 6 inches, what will be the pressure at 6 inches, 12 inches, 18 inches, and 24 inches, or just before it exhausts? A. The pressure will vary about in the inverse ratio of the volume, so that, approximately, vol. of cylinder up to point of cut-off + clearance vol. --------------------------------------------------------- vol. of cylinder at any point of expansion + clearance vol. pressure above zero, at the given point. = ---------------------------------------- pressure above zero, at point of cut-off. (23) H. T. S. asks: What size should I make the holes in the side of a fan wheel, 20 inches in diameter? Also what size should the nozzle be? A. Allow an opening of from 17 to 20 square inches at inlet and discharge. (24) E. M. D. writes: I am constructing a telephone according to directions in SCIENTIFIC AMERICAN SUPPLEMENT No. 142, using a bar magnet in place of horseshoe magnet and soft iron core. 1. Would it reduce the strength of bar magnet to cut a thread on one end of it? A. No. 2. Will a bar magnet, used in Bell telephone, lose its power to such a degree as not to work? A. Not readily. 3. Is No. 22 copper wire of sufficient size for a telephone line of 1,000 feet? A. Yes; but larger would be better. (25) S. & Y. write: We have a pair of burrs on which we grind plaster. The burrs are about 4 feet in diameter and 11/2 foot thick. We are running them as an over runner at this time, but wish to change them and make the lower burr run instead of the upper. Can a pair of burrs of the above size be run in that way, and if so, what is the maximum speed at which they can be run? A. If properly arranged, you can run them, after the change, as fast as is allowable for overrunning stones. (26) J. J. asks: Which tire makes a wheel the strongest, 1.25 x 0.50 inch iron, or 1.25 x 5/16 steel tire? A. The steel tire will be the strongest, comparing good qualities of steel and iron. (27) E. L. W. asks: Is a ton (2,000 lbs.) of first class coke equal in heat giving power to a ton (2,000 lbs.) of coal? If not, please give me the relative value of coke and coal in heat giving power? A. Calling the evaporative power of good anthracite coal 1, good bituminous coal rates at about 0.92, and coke from 0.89 to 0.95. (28) J. W. S. asks what to impregnate paper with to give it an agreeable smell while burning. A. You may try a strong ethereal or alcoholic solution of benzoin, tolu, storax, olibanum or labdanum. To burn well the paper should first be impregnated with an aqueous solution of niter and dried. (29) M. G. asks whether hydrogen and oxygen can be produced as rapidly and copiously in the decomposition of water by the galvanic battery as by the action of sulphuric acid on zinc or lead in the one case, and by heating chlorate of potassa in the other. A. Yes, with a very powerful current.