about 31 inches (0.78 metres), but with frozen liquid the fall had to be increased to about 85 inches (2.13 metre). Thus the experimental results are opposed to the acknowledged experience of practical work in the hands of common laborers. Mr. Beckerhinn found the density of the solid nitro-glycerine to be 1.735, that of the liquid 1.599, and the average melting heat to be 33.54 heat units. Thus the explosive shrinks about one-twelfth in crystallizing. ENGLISH GREAT GUNS. The largest rifled cannon in the world is a 100-ton gun, made for the Italian government by Sir William Armstrong's firm. But the English government is preparing to outdo this, and already has the plans ready for a gun of 164 tons. It hesitates, in fact, between a weapon of this size and one of 200 tons, a mass of metal which its shops are now perfectly able to handle. The meaning of the term--200-ton gun--is simply this: a tube of iron and steel of that weight, fifty feet long, having a calibre of 20 inches, and firing a shot of 3,500 or 4,000 pounds weight, with a charge of 800 pounds of powder! The human capacity for astonishment has grown perforce as the successive steps have been taken from the guns of ten and twenty tons to these weapons, which must remain huge whatever further advances are made. The character of warfare with them is best indicated by the fact that the 200-ton gun must be handled entirely by machinery. The advent of these unmanageable weapons is signalized by the invention of a hydraulic apparatus for working them. The vast shock of the recoil from the bursting of thirty-two kegs of powder--enough to throw down 1,200 tons of rock in mining--is taken up by a cylinder pierced with small holes. These holes are capped with valves, held down with a pressure of fifty tons to the square inch. When the force of the recoil exceeds this the water is forced out of the holes and the recoil thus taken up in work done. The breech of the piece is supported on a hydraulic ram, the elevation of which depresses the muzzle of the gun below the level of the deck, and brings it exactly in line with an iron tube carrying the sponge. This is run up to the base of the powder chamber, a deluge of water rushes from apertures in its head, and the bore is completely cleaned out and every spark of remaining fire extinguished. The rammer then retires, the sponge is taken off, and the powder hoisted by tackle to the muzzle, whence the rammer pushes it home, and then does the same for the shot. The shot and cartridge, weighing together about 1,350 pounds, are stored on little iron carriages, every charge in the magazine having its own carriage. The loading finished, the gun is raised, pointed, the port flies open, and the discharge immediately follows. What the result of the blow from such a projectile would be is not to be imagined. It is acknowledged, however, that in the struggle for mastery the gun has beaten defensive armor.