358 
VELOCITY AND PRESSURE INSTRUMENTS. 
will have been in motion during the sum of the above time in addition to the 
time taken by the shot in passing over the distance between the screens. Con¬ 
sequently, the observed time T' is too great by the sum of (#' + / + /' + t" ! ). 
We have also shown above that T' is too small by the quantity 6 —the time 
required to demagnetise the chronometer electro-magnet. Therefore, to ascer¬ 
tain the' true time T } we must deduct from T the quantity (O' + t' + t" + t"' — 0 ), 
which we will call t. 
We have then T=z T — t. 
Now, suppose T— 0, or, in other words, suppose the shot to cut both 
screens simultaneously, then we should have T' = t. Prom which it appears 
that t would be the time recorded on the chronometer if both currents were 
cut identically at the same instant. This we can do by using the disjunctor, 
and we thus obtain a mark, on the lower zinc tube, at a height above the 
origin equal to the space passed over in the time t, which we call the dis¬ 
junctor reading. The time corresponding to this reading must be deducted from 
the whole time recorded on the chronometer, to arrive at the time taken by 
the shot to traverse the distance between the screens. As before stated, the 
disc of the trigger can be raised or lowered so that the disjunctor reading 
can be altered (if required) within certain limits, and we can thus regulate tiie 
instrument so that the time t shall have a constant value. The value of t for 
which the velocity scale has been calculated is 0*15 of a second, and the height 
of the corresponding mark above the origin is 4*345 ins. (110*370 mill.) 
Starting with this assumption, a scale has been calculated for a distance 
between the screens of 120 ft., by means of which the velocity of the projec¬ 
tile can be at once determined without the aid of any calculation. Should 
it be necessary to place the screens nearer to one another, the velocity can b e 
found by multiplying the number read off on the scale by the fraction ^ 0 , 
B being the actual distance between the screens in feet. 
The method of calculating this scale is as follows :— 
Suppose the shot to have a velocity of 1200 ft. a second, it would take 
= 0"*1 to traverse the distance between the screens. 
The instrument will, therefore, mark 0"* 15 (disjunctor reading) -TO"*!, 
or 0"*25, and the corresponding height of fall from the origin will be 
yT' 2 = 9 X - |'? - - = 12-075 ins. 
Conversely, if the mark on the chronometer is 12*075 ins. above the origin, 
we know that the velocity of the projectile is 120 ft. a second; the dis¬ 
junctor reading being at a height corresponding to 0"*15, and the screens 
120 ft. apart. 
This calculation has been made for a series of velocities increasing from 
foot to foot for velocities from 850 to 1500 feet a second, and increasing by 
5 ft. from 1500 to 1800 ft. a second, and the corresponding heights engraved 
on the scale supplied with the instrument. 
3. Method oe Adjusting the Instrument. 
Setting up the Chronograph . 
Tor transport, the different portions of the instrument are packed in a box, 
which can be made to serve the purpose of a stand (as shown in Plate I.), 
by means of an iron tripod supplied with it. 
