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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 fig ures, 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.

vacuo.

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, the 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

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

Diameter

of bore.

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.

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

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

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21 lbs. powder, 1 shot and 1 wad, gave pressure:::

Pounds.

64,510

55,622 * 47,785

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.

At 28 in.

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At 42 in.

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12.67 186.03 927 86,750 29,300 27,800 22,420 28,400 33,850 25,050

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Constant Weight of Charge with increasing Weight of Projectile.

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"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:

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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

applied to its whole length than is required to burst it if applied to only a portion of its length; hence the pressure should diminish as the shot recedes from the breech, but not nearly as rapidly as the experiments show that it does diminish even with the largest-grained powder.

As the shot starts very slowly at the breech, and moves with constantly accelerated velocity in its course through the bore, in order to make the pressure uniform throughout, the gases should be evolved from the burning powder with a corresponding acceleration. But if the powder is granular the combustion commences on the surface of the grains and proceeds inward, constantly reducing the grains, and, consequently, the extent of the burning surface. Thus the rapidity with which the gases are evolved is retarded instead of being accelerated. Capt. Rodman conceived that if the powder was formed into hollow cylinders, to be fired wholly from the inside, the burning surface would be enlarged as the combustion progressed, and, consequently, the rapidity with which the gases were evolved would be accelerated. In order to confine the combustion to the interior of the cylinders, he moulds them together into octagonal cakes, from one to two inches in thickness, which are perforated with small holes.

The cakes are submitted to a powerful pressure in a cylinder, the plunger being armed with wires to form the holes. In practice, the axes of the cylindrical holes are parallel to that of the bore.

Capt. Rodman says that the increasing rapidity of the evolution of gas may be regulated so as to give any pressure desired along the bore, by establishing the proper relation between the number and diameter of cylindrical holes, and the thickness of the walls between them.

"The initial burning surface, and the ratio of the maximum to the mean pressure, may also be varied by varying the number and thickness of the cakes in a given weight of charge; the initial burning surface and the maximum pressure both increasing with the number of cakes, since the burning surface extends over the whole surface of the cakes.

"The thickness of walls between the cylinders should be such as to be burned through, or consumed, before the projectile leaves the gun; and for ordinary velocities we should economize in weight of charge, by making the walls of such thickness as to burn through by the time the projectile has traversed two-thirds or three-fourths of the bore, and allowing the gas to act expansively from there to the

muzzle.

"It will readily be seen, from the foregoing, that this form of cartridge gives us entire control over the rate of combustion of the charge a fact the importance of which can hardly be overrated; for, taken in connection with the hollow mode of casting cannon, it removes all limit, as regards safety, to the calibre, of which even castiron guns may be made.". Scientific American.

DOREMUS'S COMPRESSED POWDER.

The idea has been suggested by Prof. R. O. Doremus, the wellknown chemist of New York, that gunpowder for projectile purposes

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