362 
VELOCITY AND PRESSURE INSTRUMENTS. 
the registrar falls must be diminished by raising the disc of the trigger; if 
below, the disc must be lowered. In the former case it is turned in the con¬ 
trary direction to the sun, and in the latter case with the sun. 
The arrangement by which the amount of alteration in the height of the 
disc is regulated has already been pointed out when describing the trigger 
(p. 356). When the height of the disc has been regulated for some pre¬ 
vious experiments the reading will not vary on another occasion more than a 
few tenths of a millemetre, and this can be (at once) corrected by turning the 
disc, in the proper direction, through the required number of divisions of the 
circle. 
4. Method of taking Velocities. 
The instrument is prepared for measuring velocities in the same manner as 
for taking the disjunctor reading. First cock the disjunctor, then the trigger, 
and afterwards suspend the chronometer and registrar. Before suspending 
the rods, however, it is advisable, in order to prevent the possibility of 
errors in measuring the indents given by different rounds, to make marks 
round the lower edge of the upper zinc tube, about ^■ th of an inch apart, 
and to turn the tube after each round so as to bring these marks successively 
opposite the line ah, on the centre ring c (Plate I.) Equidistant lines 
are thus obtained, upon which the marks of successive rounds will be 
registered. 
The same may be done with the lower (or disjunctor) tube. By this means 
each tube can be made to register about twenty indents at each end, and can 
be turned end for end when the circle at one extremity is completed. 
An indent having been obtained on the upper tube by firing a projectile 
through the screens, the velocity may be ascertained in two ways:—1st, by 
measuring the height of the mark above the origin, and calculating the time 
and corresponding velocity from the tables; 2ndly, by measuring the velocity 
on the scale adapted for that purpose. 
The former method is the more accurate, while the latter takes less time, 
and for all ordinary purposes indicates the velocity within sufficiently narrow 
limits. 
5. Method of correcting Irregularities. 
When the foregoing directions for adjusting and regulating the instrument 
are adhered to, not only do successive disjunctor readings taken between the 
rounds agree within very narrow limits, but the readings generally remain con¬ 
stant from one round to another. It is therefore sufficient, when accus¬ 
tomed to the chronograph, in order to ensure its regularity, to take a reading 
of the disjunctor after every three or four rounds. 
The operator must judge from the regularity of these readings whether he 
should repeat them, and whether it may be necessary to readjust the instru¬ 
ment. 
The following directions will assist him :—If the reading is too high, or 
too low, repeat it, and if the difference remains constant, and is small, the 
height of the disc of the trigger need only be altered. Should, however, the 
error be considerable, indicating that there is a variation in the force of one 
of the magnets, this force must be regulated. 
