Upon cooling, the jacket exerts a constant and powerful force of compression, which counteracts the outward pressure of the force of explosion. The longitudinal strain is less dangerous than the other, and is usually counteracted by an interlocking of some of the cylinders or hoops, to which the strain is transmitted from the breech-plug. The art of building up guns has been of slow growth, the first efforts in this direction having been made by Sir W. G. Armstrong nearly half a century ago. The weight of the projectile of the present 16-inch gun in the United States service is 2370 pounds; the charge of powder weighs 1060 pounds, and the extreme range is more than 14 miles. The cost of each shot is $450, and when we consider that this does not include the wear and tear of the gun, it is evident that money has become more than ever before “the sinews of war.” Not less remarkable than the improvement in cannon is the improvement in mortars. These mortars are very unlike the clumsy weapons of that name manipulated by hand-spikes, which were known in our great war. They are now mounted on a platform which turns on rollers. They are elevated or depressed by a mechanical appliance, are loaded at the breech, are accurately rifled, and can drop their projectiles on the decks of hostile vessels at a range of six miles. They are placed in groups of four, each in a separate pit, some batteries containing as many as four groups, or sixteen mortars. In all important sea-coast batteries both guns and mortars are so arranged as to be fired by electricity, either singly or in volleys. A dynamite gun has been devised by Captain Zalinsky for the purpose, as the name implies, of throwing a projectile containing dynamite. Attempts to fire dynamite projectiles by means of powder have thus far failed. In the Zalinsky gun the propelling power is compressed air. The projectile contains from fifty to sixty pounds of gelatine dynamite, the explosion of which is terrific. Excellent results have been obtained with Zalinsky’s gun up to a range of 2000 yards, but as this is insignificant in comparison with the enormous range of high-power cannon using powder as a charge, the dynamite gun is still a weapon of limited usefulness. Although the dynamite gun has not as yet fulfilled the desired requirements as to range, promising experiments have been made in firing shells charged with high explosives from mortars using charges of powder, and it is probably a question of only a short time before means will be found for successfully firing dynamite in a similar manner.