Hints to Correspondents Names a nd Addre .... must accompany all letters, or no attention will be paid thereto . This is for our information and not for publication. Reference,. to former articles or answers should give date of paper and page or number of question. Inquiries not answered in reasonable time should be repeated; correspondents will bear in mind that some answ ers 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. B u ye I S \v ishing to purchase any article not advertised in our columns \v ill be furnished with addresses of houses manufacturing or carrying the same. Special \Vritten Information on matters of personal rather than general interest cannot be expected without remuneration. Scie ii 1 i fie A m er i e a n Supplements referred to may be had at the office. Price 10 cents each. ISo ok referred to promptly supplied on receipt of price. IHiiicral sent for examination should be distinctly marked or labeled. (6663) J. W. D. asks : In a tug of war ten men of equal strength are pulling upon each end of a rope. Is the strain upon the center of the rope equal to the pulling pow er of the men at one end or both ends of the rope ? A. One of the laws of mechanics, is, that antagonistic forces balance each other in proportion to their value. They are therefore not cumulative and cannot produce a greater strain In the connecting medium than is due to the greater of the forces. (6664) E. J. T. writes : Tb e walls of our church are built of brick, thirteen inches thick, rough cast \v ith mortar on outside and plastered on inside. The inside walls are frescoed. When we have several days nun the dampness goes through the walls, and thus ruins the frescoing. Can you give us any receipt for a preparation to go on the outside of the walls to prevent this? A. The best treatment of the outside surface of your church is to e it a thorough coat of boiled linseed oil, brushed on, and left for a few days to dry. Then if the color is objectionable, paint with any desired color. (6665) V. M. B. says : Please inform me through your columns the necessary area of a parachute to support a man who \\-eighs 200 pounds. A. The size or area of a parachute for dropping to the earth depends upon the velocity at which a person can land with safety . For 200 pounds at the rate of 5 miles per hour an area of l,o00 square feet, or about 45 feet diameter, will be required for the necessary resistance to the air at that speed. If a landing can be safely made at a velocity of 8 miles, or about 11 feet per second,an area of 615 square feet, or only 28 feet diameter, will be required. Parachutes should, be umbrella shaped, as that shape affords the greatest resistance. (6666) J. V. V., Manheim, Pa, asks: That is the present greatest elongation, east and west, of the north star ? Its declination and azimuth compared with true meridian and variation of the compass, whether east or west ? And, also, whether this variation IS increasing or decreasing ? A. The elongation of Polaris for the present year for the fortieth parallel is 1 38 1, with a yearly decrease of fourtenths of a minute . Polaris is on the meridian when Mizar, the second star from the end m the handle of the Dipper, is vertical with it, and is above the pole when the Dipper is below, or Polaris is on the farther side of the true pole from Mizar. The variation of the magnetic needle at your place for tliis year is about 5 30 west, increasing about four minutes per year.