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.