Page:Popular Science Monthly Volume 6.djvu/746

726 enters the gun, its lower extremity being in contact with, but insulated from, a sharp cutting edge, so arranged in the bore of the gun that the passage of the shot would force it down upon the wire and destroy the insulation. Each of the wires is connected with the secondary wire of an induction coil. The recording apparatus consists of a series of disks of polished silver, coated with lamp-black, and made to revolve simultaneously by the action of a falling weight and multiplying wheels at a very high velocity. One of these disks corresponds to each of the wires, the end of which is placed in a small discharger close to its circumference. On firing the charge, the shot cuts the insulation of wire after wire in rapid succession, and as each is cut a current passes, and a spark darts from the discharger to the edge of the revolving disk, striking off a speck of the lamp-black, and leaving the bright silver bare. Now, supposing the velocity of the circumference of the disks to be 1,000 inches per second, and the mark of the electric spark on the second disk to be one inch farther on than that upon the first, this would show that the shot took the $1⁄1000$ part of a second to pass from the first wire in the gun to the next. Similarly, if the distance between the marks on the first and last disks were five inches, this would indicate that the time the shot took to traverse the whole length of the gun was five-thousandths, or $1⁄200$, of a second. In reality, the time is even shorter than this. In the 10-inch gun, a 300-pound shot, with a charge of 43 pounds of powder, passes down the bore in something less than the $1⁄220$ part of a second. So delicate is this apparatus that, by dividing each inch of circumference of the disks into thousandths with the help of the vernier, the $1⁄1000000$ part of a second would become an appreciable quantity.

It is found, by careful experiment with these appliances and the crusher gauge (by which pressure is estimated by the compression of a copper cylinder placed in the bore of the gun), that the denser the powder is the slower it burns, giving a lower initial velocity to the shot, and exerting a smaller strain on the gun. As an instance of the great differences caused by the smallest variations in density, we give the following results of an experiment with the 10-inch gun, with a charge of 70 pounds:

Here an increase of .05 in density reduced the velocity by forty-two feet, and the pressure by eight tons. This shows the importance of obtaining a uniform density in the manufacture. For this purpose it is not sufficient to use a uniform pressure in the press-house, as even then the density of different pressings will vary on account of the changing