14 
MINUTES OF PROCEEDINGS OF 
purpose one extremity of a rod is fastened to the butt, and the other to the 
plate, by which means the index is drawn back by the rod, which follows 
the movement of the butt, and no longer describes the arc in the circum¬ 
ference of the wheel. 
The motion of the wheel, the distance of the muzzle of the gun to the 
butt, and the arc described by the index being known, the initial velocity 
of the projectile can be calculated.* 
It need not be remarked that this primitive arrangement had many 
disadvantages and sources of error. 
?. Col. Debooz, of the French Artillery, about the year 1838 proposed 
to measure the velocity of a projectile by an apparatus founded upon the 
free fall of a body (Fig. 4). At about 50 yards from the gun is placed a fixed 
screen, before which, and extremely close to it, a moveable screen is 
suspended by means of a fine thread, the other extremity of which passes 
over two pulleys, one near the screen, the other over the muzzle of the gun, 
and is fixed to a weight. The gun is fired, the projectile cuts the thread, 
and the moveable screen begins to descend; the two screens then are per¬ 
forated by the bullet almost at the same instant; the perforations show 
how far the moveable screen has fallen, and consequently the time which 
has elapsed from the cutting of the string till it was pierced. This time 
ought to correspond to that employed by the projectile in passing the 
50 yards. It was tried at Liege in 1840, and gave velocities too great. 
The error is attributed to the friction of the thread on the pulleys, the 
fall of the screen not coinciding with the instant of the cutting of the 
thread. The gas, also, was likely to burn the thread before the shot had 
reached it. If h is the height or distance through which the moveable 
screen falls, then 
Ballistic Bendnlums. 
8. In order to avoid the difficulty experienced in measuring the time 
of flight of a body in rapid motion, the idea was conceived of lessening the 
velocity by firing the projectile against another body heavier than itself, to 
which it would communicate the quantity of motion that it lost; the 
velocity is thus reduced in the ratio of the weight of the heavier body, plus 
the projectile, to the weight of the projectile. 
* Tide “Treatise on Gunpowder and Fire-arms/’ p. 86, by General D’Antoni. Translation by 
Captain Thomson, R.A. 
f Traite de Ballistique, par M. Didion. 
