VELOCITY AND PRESSURE INSTRUMENTS. 
357 
A very simple relation exists (as willbe # seen hereafter) between the height of 
this indent and the velocity of the projectile. It is evident that the time which 
elapses after the fall of the chonometer before the registrar is released is the 
time taken by the projectile in passing over the distance between the screens; 
the less, therefore, the velocity of the projectile, the further in advance will 
the chronometer be, and the higher will be the indent. 
A graduated rule is used for measuring the height of the indent above 
the zero point. It is of brass, and is graduated on both edges; the upper 
edge is a scale of equidistant parts, divided into inches and tenths, reading to 
thousandths with a vernier, and is intended for use in connexion with the tables. 
The lower scale is for reading off the velocity of the projectile without any 
calculation. It is graduated in feet for a distance between the screens of 
120 ft. 
The zero point on the scale corresponds with the “ origin ” or the point at 
which the knife marks the zinc if the trigger is set in action when the 
chronometer is at rest. 
The rule is fitted at the zero end with a jointed piece having a slightly 
conical projection, which enters into a recess in the bob of the chronometer 
when applied for measuring the marks. Care must be taken not to injure 
this portion of the scale or the measurement may be rendered inaccurate. 
2. Theory oe the Instrument. 
As stated above, if the trigger be set in action when the chronometer is at 
rest, a mark will be made by the knife on the zinc, which point we will call 
the “ origin ” as it is the zero point from which the height of fall of the 
chronometer must be calculated. 
Let II be the height above the origin of the mark obtained by firing' a 
projectile through the screens. Since the chronometer follows the law of the 
fall of heavy bodies, 
n= 
will be.the time it was in motion before receiving the impression. Now, T 
would be the time required by the projectile to traverse the distance between 
the screens, supposing that the chronometer commences to fall the instant the 
projectile passes through the first screen, and further, supposing that it is 
struck by the knife at the precise instant the shot cuts the second screen. 
Lut this is not the case. In fact, after the rupture of the first screen, a 
certain time, 0 , elapses before the electro-magnet is demagnetised sufficiently 
to free the chronometer; the movement of the chronometer will therefore be 
delayed, and the observed time consequently diminished, by the quantity 0. 
Again, some time elapses between the cutting of the second screen and 
the moment when the knife reaches the chronometer-—viz., the time required 
for the following operations :— 
O' for the demagnetisation of the electro-magnet supporting the registrar. 
V for the fall of the registrar to the disc of the trigger. 
t" for the disengagement of the catch. 
I'" for the knife to pass over the horizontal distance which separates it 
from the chronometer. 
INTowj it is evident that the chronometer, before it is struck by the knife, 
