A short kind of a cannon, called a car ronade, is much in naval use we have some that throw balls of near 70 lbs. their purpose is chiefly for close attacks, when their effects are dreadful : these slide in grooves on a bed carriage. The pieces used for throwing shells, which are hollow balls filled with powder that explode when the fuse burns into them, are howitzers and mortars ; the former are mounted in every respect similar to cannon, but are very short, and chamber. ed. These throw either shells or grape with great effect. The mortar is always fired at an elevation of 45 degrees from the horizon, and its range, i. e. the dis tance at which the shell is to fall, is de termined by putting a greater or less charge of powder into the chamber, shells for mortars sometimes measure a diameter of 21 inches, but those for how-, itzers rarely exceed 11 inches, and ge nerally are from 41 to Si, or thereabouts The point blank range of a cannon is that distance at which the shot cuts a line, supposed to be drawn parallel with the surface of earth, at a distance equal to the height of the chase of the cannon when horizontal. No shot goes in a right line from the muzzle to the object, but forms a curve often many yards above the horizontal line. The point blank dis tance is according to the calibre of the piece, and the proportion of powder, and its quality, used for a charge ; we may however, state the ranges to be from 40C to 1000 yards.
Mortars will throw shells more than a mile. The carriage of a mortar is a large horizontal bed of timber, strongly clamp ed together, and placed on loose sand ; it should be perfectly level. The breech
of a mortar is round, and rests in a hol low made in the centre of the bed ; its muzzle is held up by a curved iron stay, which being acted upon by a screw gives the mortar more or less elevation : the trunnions are close to the breech, and move upon the bed.
We shall conclude this article with a short description of the method of cannon boring.
Fig. 1. Plate cannon, &c. in an eleva tion of a machine for boring cannon, and fig. 2. is a plan of it; the same references are used in both figures : A is a cast iron frame to support the bearing for an iron shaft, B, turned by a steam engine, or water wheel ; this has a square box on its end, into which a square knob cast on the end of the gun is fitted by screws ; the mouth of the gun is supported on an iron frame, D, sliding on the two bed beams, E, E, and can be fixed at any place by screws ; it has also screws to elevate or depress the brass which forms the bearing for the gun ; F is the boring bar, fastened at its end to a large block, G, running on the bed beams with small wheels : H is a rack fastened by its ends to puppets wedged on the bed, passing through the block G: a pinion which works in this rack is attached to the block G, and its spindle has a wheel, I, with pins projecting from it : K is a bar going between these pins, and carrying a weight which turns the pinion, and forces the block G, and the boring bar, towards the gun. When the weight reaches the ground it must be lifted up, and its lever, K, hooked between two fresh pins of the wheel.