accumulating at the bottom may be forced up the inner pipes by the steam pressure and escape at the top. The vessel is charged through the manhole, e, and the hopper, c, provided with a perforated cover, and is discharged at the bottom by the valve, f, shown in Figs. 2 and 3. The upper part of the boiler serves as a steam dome, and the pressure on the liquid in the receiver and on the water in the annular space is thereby maintained uniform. The necessary fittings for showing the pressure in the vessel, water level indicator, safety valve, cocks for testing solutions, etc., are of course added to the apparatus, but are not indicated in the drawing. The arrangement appears to us to possess considerable merit, and we shall refer to it again on another occasion, after experiments have been made to test its efficiency.--_Engineering_. [Illustration: IMPROVED DOUBLE BOILER.] * * * * * THE GARDNER MACHINE GUN. [Illustration: FIG. 1.--SINGLE BARREL GARDNER MACHINE GUN.] The mechanism by which the various functions of loading, firing, and extracting are performed is contained in a rectangular gun metal case, varying in dimensions with the number of barrels in the arm. In the single barrel gun the size of this case is 14 inches in length, 5½ inches in depth, and 2½ inches in width. The top of the box is hinged, so that easy access can be had to the mechanism, which consists of a lock, the cartridge carrier, and the devices for actuating them. In the multiple barrel guns, the frames which, with the transverse bar at the end, hold the barrels in place, form the sides of the mechanism chamber, in the front end of which the barrels are screwed. The mechanism is actuated by a cam shaft worked by a hand crank on one side of the chamber. By this means the locks are driven backward and forward, the latter motion forcing the cartridges into place, and the former withdrawing the empty cartridge case after firing. The extractor hook pivoted to the lock plunger rises, as the lock advances, over the rim of the case, but is rigid as the lock is withdrawn, so that the action is a positive one. The cartridges, which are contained in a suitable frame attached to the forward part of the breech chamber, pass through openings in the top plate of the latter, an efficient distribution being secured by means of a valve having a transverse motion. Each cartridge as it falls is brought into the axis of the barrel and the plunger, while the advance motion of the lock forces them into position. In the five-barrel gun illustrated by Fig. 3 the cartridge feeder contains 100 cartridges, in five Vertical rows of 20 cartridges each, and these are kept supplied, when firing, from supplementary holders. Fig. 1 shows the portable rest manufactured by the Gardner Gun Company. It consists of two wrought iron tubes, placed at right angles to each other; the front bar can be easily unlocked, and placed in line with the trail bar, from