R. B. C., of Pa., says: "Here is a proposition in geometry which I would like to see demonstrated theoretically by one of your correspondents. The side of a regular heptagon is equal to half the side of an equilateral triangle inscribed in the same circle. The mechanical construction is very simple and will be found useful. I discovered it some years ago and am not aware of its ever having been in print." F. H., of Mich., asks "if sal-soda will scale a boiler?" H. N. Winans, 11 Wall street, N. Y. replies that in some waters it is partially effective but at the expense of the boiler, with a certainty of foaming and corrosion. The most reliable and positively uninjurious remedy for incrustations is his anti-incrustation powder--in successful use for 12 years past. T., of R. I., speaks of the famous mechanical horse shown at the Paris Exposition which is said to have accomplished with its rider a little over an English mile in fifty seconds, and asks what is the motive power. As it is said that the French Government took possession of the machine and preserves its mechanical construction a secret, we know no more about it than about the much vaunted Napoleon cannon. S.S., of N. Y.--"Please give the ingredients of the composition used for tipping matches." Different manufacturers employ different materials and in varying proportions; the mixture of phosphorus melted and stirred up with thin glue is sufficient, although some add a quantity of powdered glass, niter, chlorate of potash, sulphur, etc. The phosphorus, however is the light-producing material. R.S.B., of N.Y., alluding to the inquiry of S.W.P., in No. 23, for a waterproof paste. "Calico printers when they wish to leave white figures on a dark ground use what they term a 'resist paste' to cover such places as are designed to be unaffected by the dye. If the ingredients of this paste were known it might be what S.W.P., desires." This "resist paste" is 1 lb. of binacetate of copper (distilled verdigris), 3 lbs. sulphate of copper dissolved in 1 gal. water. This solution to be thickened with 2 lbs. gum senegal, 1 lb. British gum and 4 lbs. pipe clay; adding afterward, 2 oz. nitrate of copper as a deliquescent. M.A.H, of Vt.--"I have a surplus of water power and desire to know the probable cost of the apparatus for producing the electric light, with a view of employing my surplus power in that direction." A serviceable magneto-electrical machine for giving light is quite expensive. * * * * * Business and Personal. _The charge for insertion under this head is 50 cents a line_. * * * * * Parties in want of Fine Tools or Machinists' Supplies send for price list to Goodnow & Wightman, 23 Cornhill, Boston, Mass. Pattern Letters and Figures for inventors, etc., to put on patterns for castings, are made by Knight Brothers, Seneca Falls, N.Y.