may not be got from hydraulic propulsion, but it can only be had by making the quantity of water sent astern as great as possible and its velocity as small as possible.  That is to say, very large nozzles must be employed.  Again, provision will have to be made for sending the water through the propeller in such a way that it shall have as little as possible of the motion of the ship imparted to it.  But as soon as we begin to reduce these principles to practice, it will be seen that we get something very like a paddle wheel hung in the middle of the boat and working through an aperture in her hull, or else a screw propeller put into a tube traversing her from stem to stern.

We may sum up by saying that the hydraulic propeller is less efficient than the screw, because it does more work on the water and less on the boat; and that the boat in turn does more work on the water than does one propelled by a screw, because she has to take in thousands of tons per hour and impart to them a velocity equal to her own.  Part of this work is got back again in a way sufficiently obvious, but not all.  If it were all wasted, the efficiency of the hydraulic propeller would be so low that nothing would be heard about it, and we certainly should not have written this article.—­The Engineer.

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THE NEW ARMY GUN.

The cut we give is from a photograph taken shortly after the recent firings.  The carriage upon which it is mounted is the one designed by the Department and manufactured by the West Point Foundry, about six months since.  It was designed as a proof carriage for this gun and also for the 10 inch steel gun in course of construction.  It is adapted to the larger gun by introducing two steel bushing rings fitted into the cheeks of carriage to secure the trunnion of the gun.

The gun represented is an 8 inch, all steel, breech-loading rifle, manufactured by the West Point Foundry, upon designs from the Army Ordnance Bureau.  The tube and jacket were obtained from Whitworth, and the hoops and the breech mechanism forgings from the Midvale Steel Company.  The total weight of the gun is 13 tons; total length, including breech mechanism, 271 inches; length of bore in front of gas check, 30 calibers; powder space in chamber, 3,109 cubic inches; charge, 100 pounds.  The tube extends back to breech recess from muzzle, in one solid piece.  The breech block is carried in the jacket, the thread cut in the rear portion of the jacket.  The jacket extends forward and is shrunk over the tube about 871/2 inches.  The re-enforce is strengthened by two rows of steel hoops; the trunnion hoops form one of the outer layers.  In front of the jacket a single row of hoops is shrunk on the tube and extends toward the muzzle, leaving 91 inches of the muzzle end of the tube unhooped.  The second row of hoops is shrunk on forward of the trunnion hoops for a length of 38 inches to strengthen the gun, and the hoop portion forms three conical frustums.  The elastic resistance of the gun to tangential rupture over the powder chamber is computed by Claverino and kindred formulas to be 54,000 lb. per square inch.