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C.H.G., of N.Y.-To make pure nitrate of silver, dissolve pure silver in pure nitric acid, evaporate the solution to dryness, or, if crystals are preferred, evaporate until the solution is sufficiently concentrated to form crystals. If you can not get pure silver, you may purify it by dissolving coin in nitric acid, filtering the solution and precipitating the silver in the form of a chloride by hydrochloric acid. Next wash the precipitate with hot water until the washings cease to redden litmus paper. Next mix the pure chloride of silver while yet moist with its own weight of pure crystallized carbonate of soda, place the mixture in a covered porcelain crucible and heat very gradually until the fusing point of silver is reached. The reduced silver will be pure and may be removed by breaking the crucible. Wash the button thoroughly with hot water to remove the flux. In dissolving the pure silver thus obtained in nitric acid, it is better to use an excess of acid; the excess will be driven off by heat in evaporation.

G.B., of Iowa.-Nominal horse power is merely a conventional expression for diameter of cylinder and length of stroke, and does not apply to the actual power of the engine. It is found by multiplying the cube root of the stroke in feet by the square of the diameter in inches and dividing the product by 47. This rule is based upon the postulate established by Watt, that the speed of a piston with two feet stroke is 160 feet per minute, and that for longer strokes the speed varies as the cube roots of the length of the stroke. It is needless to say this rule is not observed in modern practice, yet the expression, nominal horse power, is like many other relics of past time still retained. The above rule does not apply to high pressure engines. For such engines Bourne has given the following rule: Multiply the square of the diameter of the cylinder in inches by the cube root of the stroke in feet, and divide by 15.6. The real power of an engine is estimated from the mean effective pressure in the cylinder--not the boiler--and the speed of the piston. Your data are insufficient to determine the horse power of your boiler. The horse power of boilers is estimated from the extent of heating surface when the grate and all other things are correctly proportioned, but with them as with engines, only actual test will positively determine it. The pipe you mention ought to be enlarged as proposed.

W.H.R., of Mass.-Pressure acts independently of the mode of application. A tun laid upon the head of a wedge would produce the same effect as though it were applied through toggles. When, however, a weight is dropped its effect increases as the square of its velocity.

J.B., of N.Y.-We recommend you to get "Appleton's Dictionary of Mechanics." Also send for descriptive catalogue to Henry Carey Baird, Philadelphia, from which you will be able to judge for yourself what works are suited to your requirements.

T.D.H., of Mass.-Ammonia, in a weak solution, may be used to cleanse the scalp, but is not recommended for the purpose. Borax in solution is better. The supposed preservation of the color of the hair by its use is a mistake.

F.B.H., of Ill.-So far as we know, nothing better than the flax seed bag has been discovered for packing the lower end of tubes in artesian wells. We have never heard of any trouble arising from the method and think you will have none.

L.G. of Mass.-Express the decimal ratio of the diameter of a circle to the circumference to which you refer, as a mixed vulgar fraction, and you will have what you ask for, if we understand your query.

A.H.S., of Sandwich Islands.-We know of no substance that in our opinion, could be used advantageously to paint the interior of sheet-iron evaporating pans for concentrating cane juice.

L.B., of Wis.-We would be glad to assist you but the data you furnish are not sufficient. The accurate solution of such a problem involves the higher mathematics.

A.H.M.-All animal and mineral oils are destructive to rubber. Linseed oil will not dissolve it. Oils should not be allowed to get on rubber belting.

T.W.J., of Pa.-For your rollers try some emery mixed in a solution of gum shellac in good alcohol.

E.B., of Mass.-The patent can be corrected by reissue.

J.M.T., of Ind.-To find the proper area for a safety valve port, when the evaporating surface is properly proportioned to the engine power, multiply the square of the diameter of the piston in inches by the speed in feet of the piston per minute, and divide the product by 375 times the pressure on the boiler per square inch. Having decided upon the length of the lever, the distance of the valve stem from the fulcrum, and the point from which the weight will be suspended, the weight necessary will be found by multiplying the area of the valve port in inches into the pressure per square inch in the boiler in pounds, and this product into the distance of the center of the valve stem from the fulcrum in inches, and dividing the product thus obtained by the distance from the fulcrum to the point of suspension of the weight in inches. The quotient will give the weight in pounds.

A.K.S., of Ohio.-The inclination of the poles of a planet to the plane of its orbit, determines its zones and also its seasons. The inclination of the earth's axis is twenty-three and one half degrees. This places the tropics the same distance each side of the equator, and the polar circles the same distance from the poles. The torrid zone is therefore forty-seven degrees wide, and the temperate zones each forty-three degrees wide. As the planets vary in their inclination of their axis to the planes of their orbits, it follows that their zones and seasons differ from those of the earth.

W.H.C., of Texas.-The teeth of a circular wood saw to be driven by foot-power, should be not larger than those of the ordinary hand crosscut. The fly-wheel ought to have a rim weighing from eighty to one hundred pounds, and it should be, for a 12-inch saw, not less than a foot in diameter. It should be placed on the saw arbor. The belt should not run on the fly-wheel, but on a special pulley, and the treadle and crank motion should be so adjusted that the foot will move through an arc of from 10 to 12 inches.

A.H.B., of Pa.-We advise you to use a battery in coating the small gray castings, of which you write, with copper. It will be all the more satisfactory in the end. The best polishing material to put in with them in the tumbler we think would be leather cuttings and sweepings. They will not need returning to the tumbler after being coppered. We recommend you to get "Byrne's Practical Metalworkers Assistant," published by Henry Carey Baird, Philadelphia.

J.H.G., of Tenn.-Don't put oil in your boiler to prevent incrustation. It will not probably do any good, and it will cause much foaming, while besides that it is a waste of heat, it is injurious to engines.

S.S.R., of Tenn.-No ammoniacal engines are, so far as we are aware, running in this country.

C.E.C., of Ohio.-The varnish for patterns is common shellac varnish. It is sometimes made black by lampblack.