We have now the pressure at the surface of contact, and we require the shrinkage. In (1), we substitute this pressure for p and the radius of contact for r. Next we place p = 0, and r equal to the radius of the bore. Solve for c1 and c, (they have already been found, however,) and substitute in (4), making the radius of contact. The result is the strain, and taken with a negative sign is the compression (per unit) of the outer diameter of the inside portion. In (1) we substitute the pressure at the surface of contact for p, and the radius of contact for r. Next o for p and the external radius for r. Solve for c, and c. Substitute in (4), taking r the radius of the surface of contact, and the result is the extension per unit of the inner diameter of the hoop. Add this extension to the preceding compression and multiply by the diameter, and the result is the required shrinkage. Various other problems might be solved in the same way but will not be gone into here. Our endeavor has been to avoid the usual nomenclature of books on the elastic strength of guns, which is something appalling, by the use of equations that will keep what we are doing clearly before us. Certain minor operations are duplicated in the description, which would not really be necessary with actual numerical cases, and it is thought that this adds to the clearness. It may be remarked that the pressures at the surfaces of contact of a built-up gun when the gun is at rest may be used to find the shrinkages, in exactly the same way as shown in this note using the maximum firing pressure. In fact, the corresponding pressures at these surfaces under any given conditions* may be used. If two adjacent cylinders in a built-up gun have the same elastic limit, the most advantageous intermediate radius will be a mean proportional between the other two radii. The proof of this will not be given here as it would unnecessarily complicate what is intended only as a brief summary of the subject. Other considerations frequently interfere with this. For example, the jacket of a modern gun is made of sufficient thickness to withstand the longitudinal pull between trunnions and breech block. If P. and R are the elastic strength of the gun and the radius of the exposed * Attention is called to this by Captain J. P. Story, U. S. Artillery, in his Elastic Strength of Guns. It may be stated here that the notation used for strains is his also. nose of the breech block respectively, the elastic strength of the metal and R1 and R, the interior and exterior radii of the jacket, OT(RR) should be equal to or greater than PR, so that no unit area may be exposed to a lengthwise pull greater than the elastic strength of the metal.* * It may be necessary to consider the strains. Tangential stresses decrease and radial pressures increase the longitudinal strain, so that the longitudinal stress, as here used, should give fair results. If strains are considered, they should be determined originally on the condition that the longitudinal stress of the jacket is the one here used. PROFESSIONAL NOTES. LEAK ARRESTERS FOR SHIPS. Experiments made by Mr. Colomès, a French inventor, with cellulose applied to holes in the hull, induced the French Government to adopt his device to be used on board its war vessels. The apparatus for applying the cellulose to the hole is extremely simple. It is composed of a steel rod, threaded on a part of its length, at the end of which is pivoted an iron piece, which, when at right angles to the rod, has the appearance of a pickaxe, one of the arms of this cross piece being heavier than the other. This cross piece has fixed to it an oval piece of flat iron covered on both sides with thick felt. A small conical bag, filled with cellulose and having a hole through its center, can be slid on to the rod. Back of this bag is applied a large washer, which is held in place against the bag by a nut which is pushed down the rod to the threaded part, where it engages the screw. When a leak has been located any man can seize a leak stopper corresponding approximately in size to the width of the hole. Then holding it with the lighter end of the pick toward him, so that the pick and oval plate lie alongside the rod, he can introduce it into the hole. He can avoid the rush of the water by standing to one side. As soon as the pick has passed through the plating the heavier end descends and the pick places itself across the hole while the pressure of the outside water forces it against the side of the vessel and throws the pick arm across the opening, so, resting on the plating around the hole, it affords a point of support, while the felt covered plate reduces the leak very much and makes easier the next operation, which consists in slipping the bag of cellulose, washer and nut over the rod, screwing down the nut till the bag of cellulose is compressed against the hole. The cellulose bag fills up all parts of the hole, no matter how irreg ular, as the great value of the cellulose consists in its absorbing water and greatly increasing its volume. This elastic mass makes a tightly applied mat over the hole, which cannot be accidentally disturbed or displaced. Should the hole not be more than 10 inches wide and several feet long, a number of leak stoppers can be used side by side so as to gradually fill the hole. Three sizes of arresters are used: No. I for holes from 1 to 3 inches, No. 2 for holes from 3 to 6 inches and No. for holes from 6 to 10 inches. In order to practically demonstrate the value of the leak arrester, the Franco-American Cellulose Company of 831 Arch street, Philadelphia, erected at their works a set of tanks pierced with holes of different sizes and shapes. The first experiment took place last year before a board appointed by the Navy Department and a number of naval officers and naval constructors, among whom were Lewis Nixon, chief constructor of the William Cramp & Sons Ship and Engine Building Company, and Captain Constance, naval attaché of the British Legation in Washington. The leak arresters were to be placed in three holes cut in the sides of an iron tank. The smallest hole was circular with burred edges and was 2 inches in diameter; the next was hexagonal, about 10 inches wide, its area being about 72 square inches; the third was very irregular in shape and about 21 inches long, the average width being about 5 inches and the area 85 square inches. It being understood that the stoppers are intended to be used from the inside of the ship, the tanks were supposed to represent the sea and the holes or rents were located at a depth of 10 or 12 feet below the water line, with a corresponding water pressure. The tanks were kept full by means of a pump so as to preserve the same head of water during all the tests. The time employed to effectually close the holes under a head of water of 12 feet was as follows: Another test made immediately after the above, using a water pressure of 9 feet, gave the following results: 10-inch hole....... 21-inch hole.... ..... 37 seconds. .I minute, 40 seconds. During the latest tests made three leak stoppers were placed side by side, instead of two, in order to show that any number of leak stoppers can be employed to gradually decrease the leakage until the hole is under control. It is said that after the test Constructor Nixon expressed his opinion as fol. lows: "The experiment was a signal success, and the holes were stopped in remarkably short periods. By the use of the Colomès leak stoppers and cellulose any leak in any vessel can be stopped before an appreciable quantity of water can rush in." For holes of much larger area Mr. Colomès proposes to use a cellulose mat to be applied from the outside of the vessel. This mat resembles an ordinary mattress, filled with obturating cellulose and is made in several sizes. The side of the mat away from the side next to the ship is covered with water proof cloth in order to prevent too much water from filtering through the cellulose. On the sides and at the corners rings are fixed intended to receive guiding ropes. Such ropes should always be kept in readiness on the upper deck, bent and with the slack so arranged that they will fall under the vessel so as to hang from gunwale to gunwale. These ropes are to receive the mats as soon as a leak is discovered and located. The soft pliant nature of the cellulose lining of the mat enables the pressure of the water to force it into all parts of the opening, so that every crack is filled and the inflow automatically stopped. The Franco-American Cellulose Company is now experimenting with a view to finding a non-combustible substitute for the woodwork of the cruisers and battle-ships of the navy. EXPERIMENTS ON WIND PRESSURE. The subject of wind pressure is one on which our knowledge at the present day is not only limited, but exceedingly vague, and carefully-made experiments, if but to investigate a single feature of the problem, are, therefore, of the greatest interest, and can hardly fail to add something new to our information. Mr. J. Irminger, C. E., Member of the Danish Society of Engineers, has determined, what it is believed no one before him has attempted to do, the amount of suction produced by a current of air striking a plane surface, or the surfaces of various bodies; and the results of his experiments form the subject of a paper with the above title, read before that society in the early part of last summer. These results are remarkable in showing how very large a percentage of the total effect this suction is, not only through its action on the leeward side, but on the windward as well. In fact, when the angle at which the wind strikes a plane surface is small, nothing but suction is produced. The practical importance of these experiments are evident; they throw con |