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THE POPULAR SCIENCE MONTHLY 



enters the gun, its lower extremity being in contact with, but insu- 

 lated 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 sec- 

 ondary 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 cor- 

 responds 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 t J part 

 of a second to pass from the first wire in the gun to the next. Simi- 

 larly, 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 ^i-g-, of a 

 second. In reality, the time is even shorter than this. In the 10-inch 

 gun, a 300-pound shot, Avith a charge of 43 pounds of powder, passes 

 down the bore in something less than the -%^ part of a second. So deli- 

 cate is this apparatus that, by dividing each inch of circumference of 

 the disks into thousandths with the help of the vernier, the 100 ^ 000 

 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 ob- 

 taining 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 



