on a ship's deck may be used both as a cannon and a mortar, and that a ship which carries forty guns may be said to carry forty guns and forty mortars.

Trial of the great "Rodman" Gun.-The great 12-inch rifled cannon, constructed according to the plan devised by Lieut. Rodman, U. S. A. (see Annual Sci. Dis. 1861, p. 50), for the government, has been mounted during the past summer at Fortress Monroe, and thoroughly tested. This gun is sixteen feet long, weighs fifty-two thousand pounds, and carries a solid shot weighing four hundred and twenty-three pounds. The N. Y. Tribune gives the following spirited description of the firing of this monstrous piece of

ordnance:

“First, the swab, which two men insert, cleans the interior; then comes a man with a red flannel bag filled with powder on his shoulder; it is the cartridge; and the ramrod, worked by four men, sends it home. Then comes the process of getting the shot to its place, a mass of metal that four men carry with difficulty. It is first rolled into an iron cradle or wide strap, looped with a rope, through which a piece of strong wood, several feet long, is inserted. Thus slung, the men tug the shot up the steps of the platform, some ten feet high, to the mouth of the gun. Another lift brings it on a level, and it is slid into the muzzle and pushed to its place. The gun is then elevated by means of an iron bar, the gun being nicely balanced on its trunnions. The priming wire performs its office, the fuse is inserted, and the lanyard is attached. Ready,' says the sergeant; 'Ready-fire!' says the lieutenant. A terrific crash, a sheet of flame, and the trembling of the earth, follow. At the same time there is a screech and a scream caused by the shot, a black mass which you can see, in its flight, yelling like an infuriated devil let loose from the infernal regions."

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The results of the practice with this gun demonstrate its complete success, and it is probable that government will order their construction in considerable numbers for our sea-coast fortifications. In its ranges for shot and shell, it does not exhibit any decided advantages over those obtained from the 10-inch gun, up to 10° elevation; but beyond this elevation the gain is considerable, and may be estimated at about 600 yards for the elevation of 28° 35'. With 39° elevation, and a charge of forty pounds large-grained powder, it is probable a range considerably beyond four miles might be attained. Indeed, the conviction appeared to be general among the officers who witnessed the firings, that the gun could, if necessary, bear much heavier charges, though, for all ordinary uses, it is doubtless best to keep them down to somewhat less than the above.

The Chronoscope. This name has been given to a recent English invention, which is intended to measure accurately the time of the flight of projectiles. It consists, in the first place, of a barrel, revolving by clockwork, which is cased with paper, upon which four or more pencils trace a continuous line. These pencils are in communication with a voltaic battery and electro-magnetic apparatus, and the targets, made of fine crossed wire, which may be four or more in number, according to the pencils, are placed 200, 300, 500 or 700 yards apart. When the shot strikes the first target, the current is broken

vacuo.

and one of the pencils stops marking, and so on with the remaining targets; and by means of a pendulum the records are reduced to figures, which give the initial velocity of the shot, at various parts of its flight, which is of the utmost importance, as it includes the resistance of the air, and affords practical data for the most correct calculations through larger flights. The resistance of the air has always been estimated from certain known laws, but now it may be determined by practical experiments. It is not expected that the instrument will register correctly beyond seven hundred or one thousand yards. Illustration of the Movement of Projectiles in Vacuo.· The following is a description of an invention by Col. Fox, of the British army, intended to demonstrate the parabolic theory of projectiles in The initial velocity being taken at 107 yards per second, a bar is provided with movable wires and beads at the extremities, the length of which increases as the square of the times; on placing the bar, which represents the plane of the direction of the shot, at any required angle, a beautiful parabola is produced. The instrument shows at any elevation the range of the projectile; for example, at 20° elevation the range is 700 yards, at 30° 920 yards, 45° greatest, when it registers 1050 yards. If the angle is still further increased, range diminishes in proportion, showing that forty-five degrees is the maximum elevation for the greatest range. The apparatus is also capable of showing the range of a shot when fired down from an eminence with depression; and a parallelogram arrangement is adapted to the parabolic curves, to prove that they are the result of a compound force. The Nyctoscope.. Sir W. Armstrong has described to the London Institution of Civil Engineers the principle of the Nyctoscope, an ingenious instrument designed by him for enabling the gunners to maintain a fire upon any given object after nightfall. The principle of the instrument is to render a false object in the rear, or at one side, visible upon a vertical line in a mirror, when the gun is laid upon the true object. A lamp attached at night to the false object becomes visible upon the same mark in the mirror, when the gun is in line with the true object. The vertical adjustment for elevation is effected by a spirit-level clinometer, forming part of the instru

the

ment.

RODMAN'S EXPERIMENTS IN GUNNERY.

It is perhaps well known to many of our readers that there has been in progress for several years a series of costly experiments (instituted by the U. S. War Department) to determine the best form and material for cannon, and the qualities desirable in gunpowder. These experiments have been conducted, for the most part, under the direction of Capt. T. J. Rodman, of the ordnance department, U. S. A., and by him have been recently published in an illustrated

volume.

Some of the most interesting facts developed by these experiments, and set forth at length in the volume above referred to, are in relation to the pressure in a cannon, at the time of its discharge, exerted by the gases resulting from the combustion of the powder. To meas

ure this pressure an instrument was devised, which is illustrated in the accompanying engraving.

A hole about a third of an inch in diameter is drilled through the wall of the gun to the bore, and the outer portion of this hole is enlarged to receive the end of a cylinder, a, which has a piston working within it. In the cut, b represents the portion of the cylinder

a

which is screwed into the hole in the cannon, and c is the piston, corresponding in size to the smaller portion of the hole. The gases, pressing on the inner end of the cylinder, force it outward. Its outer end is armed with a steel point, d, which is forced into a copper bar, e, to a depth depending upon the amount of the pressure. The copper bar and steel point are then placed under massive steelyards, and the force required to produce an indentation equal to that produced by the gas is accurately weighed. Capt. Rodman says that a difference of two pounds in 30,000 is plainly perceptible; “so that the indications of this instrument may be safely regarded as approximating to within 1,000 pounds of the true pressure, even for the greatest pressures exerted, and much nearer for the smaller pres

sures."

We give some of the most interesting results obtained:

-

Pressure per square inch due to Proof Charges in a 42-Pounder Gun.

Pounds.

21 lbs. powder, 2 shot and 1 wad, gave a pressure at the bottom of the bore

14 lbs. powder, 2 shot and 1 wad, gave a pressure at the bottom of the bore

64,510

55,622

Table showing the velocity of shot, in feet per second, and pressure of gas per square inch, in pounds, due to equal columns of powder behind equal columns of metal, when fired in guns of different diameter of bore, each result being a mean of ten fires.

Pressure different at distances from bottom of bore.

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་

5.13 74.44 904 36,420 15,850 8,370 6,470 6,850 8,050 6,720

9 8.48 124.42 888 67,100 21,100 17,750 14,900 29,475 20,970 22,825 11 12.67 186.03 927 86,750 29,300 27,800 22,420 28,400 33,850 25,050

Constant Weight of Charge with increasing Weight of Projectile.

"The points most worthy of note in these results are the very marked increase in pressure of gas as the diameter of bore increases, and that the indications of pressure are greater at 56 inches, 70 inches, and 84 inches than at 42 inches, especially in the 9-inch and 11-inch guns. The cause of the difference of pressure developed in these guns of different diameters of bores is believed to be mainly due to the great heat developed by the combustion of the larger mass of powder in the large than in the smaller calibre; and perhaps, also, to the different products of combustion formed under this increased temperature and pressure, and partly to the greater cooling surface in proportion to the weight of charge in the smaller than in the larger calibre."

The highest pressure observed in a cannon was 100,000 pounds to the square inch, but this was greatly exceeded in a shell. A very strong shell was cast; the exterior diameter being twelve inches, and the interior a little less than four, with an orifice only one-tenth of an inch in diameter, this orifice being the only outlet for the gas. The cavity was filled with powder, which was fired, when the instrument indicated a pressure of 185,000 pounds to the inch.

The following are some of the conclusions to which Capt. Rodman was led by experiments which we have not space to describe in detail:

-:

"Time is required for the rupture of any mass of iron, though the rupturing force may be greatly in excess of the resistance of that mass. And in the ordinary discharge of cannon the gun is subjected at each discharge to a force which would inevitably burst it, if permitted to act for any appreciable length of time; so that it may be said that cannon do not burst because they have not time to do so before the bursting pressure is relieved."

"Pressure increases in a higher ratio than that of the volume of powder; it being, for the larger charges, almost as the squares of the volumes."

INTERESTING EXPERIMENTS WITH GUNPOWDER.

When ordinary small-grained powder is burned in a cannon, the combustion is so rapid, and the gases are consequently so quickly developed and so highly heated, that an enormous pressure is produced at the breech of the gun before the ball starts from its seat; then, as the gases expand, the pressure is rapidly reduced, so that the velocity of the ball is small in proportion to the maximum pressure exerted upon the gun. It occurred to Capt. T. J. Rodman, of the Ordnance Department, U. S. A., that if the powder were made to burn a little more slowly, the pressure would be less at the breech, and would follow up the ball with more force during its passage out of the gun, thus giving greater velocity to the shot with less danger of bursting the cannon.

The first plan that he tried for producing a slower combustion of the powder was to make it in large grains, which were compressed with great force, so that they could not be permeated by the gas, and, consequently, could burn only by a gradual combustion commencing on the outside and extending inward. Powder of the same quality in every respect, except the size of the grains, was prepared by the Messrs. Dupont, the grains in one sample being all threetenths of an inch in size, those of another four-tenths, of another fivetenths, and of the last six-tenths. Capt. Rodman made a series of fires with this powder in a 11-inch gun, using the same weight of charge, 12.67 lbs., and the same cylindrical shot, weighing 183.3 lbs., at every fire. Five fires were made with powder of each size of grain, and the mean results are exhibited in the following table : —

The smallest-grained powder, three-tenths of an inch in size, produced a pressure at the bottom of the bore of 35,000 lbs. to the square inch; which was reduced to 6,700 lbs. at 28 inches from the bottom of the bore, giving a velocity to the shot of only 890 feet per second; while the powder of largest grain, six-tenths of an inch in size, though producing a pressure of only 21,000 lbs. at the bottom of bore, followed it up with 8,000 lbs. at 28 inches, and gave a velocity to the shot of 933 feet per second.

The granular form, however, is not the best for cannon powder, whatever the size of the grains. In order to give the greatest possible velocity to the shot, with such degree of pressure as may be safely employed, the pressure against the shot should continue nearly uniform throughout its passage from the gun. It should be exactly uniform were it not for the fact that a less pressure will burst a gun

if