Timing Big Projectiles

To the layrnan one of the interesting features in a big gun test is the ruethod by which the initial velocity of the projectiles is calculated. When he hears that the modern high power guns ofteu expel a shot at an initial velooity of 2,000 feet a second, a rate that if sustained would mean a mile in three seconds, he realizes the difficulty of calculating the speed. An ordnance officer must have accurate knowledge of the velocity of a projectile, that he may predict its range and penetration and determine the accuracy of the gun. It is a coruparatively easy matter, however, ■with the new instruments to calcúlate accurately the initial velocity of a projectile, and any one who is fortúnate enough to visit Sandy Hook when big guns are being tested may see how it is done. Two open frames are set up 150 feet ajpart in front of the gun. Wires are stretched back and forth across these frames, making a screen through which the shot must pass. The wires in eacb. screen form a complete electric circuit, ■which includes also an electric battery and an electro magnet. The projectile, after leaving the gun, fiies through the ■wire in the first screen, interrupting the circuit and releasing the armature of the magnet. In a space of time so small as to be hardly conceivable the projectile has covered the distance between the two frames and pierced the wire in the second, interrupting its electric current and feleasing the armature of its magnet, as in the first case. The interval betweeu the drop of these two armatures represents the time spent by the projectile in traveling 150 feet, This time is indicated by the chronograph in the laboratory near by. Wires run frona each screen to the laboratory, which is fitted up with batteries and switchboards. The armature of the first electro magnet is an iron rod about 3 feet long, which is suspended vertically. This rod falls when the second electro magnet ia placed a little below the first, and when it is released it acts as a knife, and, striking the side of tho falling rod, makes a slight mark. The distance of this cutorniark from the end of the rod indicates the distance through which the rod has dropped while the projectile is passing from one screen to another. This forms the unit for the calculatioii of the projectile's velocity in feet per second. New explosives and high power guns have increased the initial velocity of projectiles wonderfclly in the last few years. Projectiles may be expelled now with a forcé that will make them effective at a ranee far bevond ordinary eyesight. ■