VELOCITY AND PRESSURE INSTRUMENTS. 
361 
one another, remain with their points only in contact, and place the hob in 
its proper position by moving the first finger of the right hand, upon which 
it still rests. This done, withdraw the support of the right hand by 
lowering it vertically, when the chronometer will remain suspended in its 
proper position. If there should be any vibration, it will soon cease from 
the friction against the rest. 
The registrar is suspended in the same manner, but the chronometer is 
always placed in position first . 
Difficulty is sometimes experienced by beginners in suspending the parts 
of the chronograph; minute details have therefore been given as to the best 
way of doing so, and these should be strictly adhered to. 
The other operations are exceedingly simple. 
To cock the disjunctor, so as to establish the currents, press the milled- 
headed screw £ (Plate II., Pig. 9) with the centre finger of the right hand, 
until the spring is held by the catch x. 
To break the currents, press the catch, x, with the forefinger of the right 
hand, the thumb being placed against the support, y. 
In cocking the trigger, care must be taken not to disturb the level of the 
instrument; consequently the left hand only is used, the fingers being placed 
against the support of the tube, L (Plate I.), and the spring drawn back with 
the thumb until it is held by the claw of the lever. 
The trigger must always he cached before attempting to suspend the 
chronometer . 
(3) 'Regulating the disjunctor reading. —As we have said before, this 
reading should represent a time — O''* 15, and the mark should consequently 
be 4*345 ins. above the origin. This height is shown on the scale by a 
special mark, called “ disjonctor.” To facilitate the operation, commence by 
tracing on the small zinc tube a circle at the required height. Por this purpose 
fasten the vernier by means of the clamping screw, with the index opposite the 
line marked “ disjonctor.” Place the chronometer flat on a table with the 
numbered face next the body, and apply the rule to it, by inserting the 
conical point of the hinge in the recess of the bob, allowing the index of the 
vernier to rest on the zinc. Support the end of the rule in the right hand, 
and, with the left, turn the tube, taking care to keep it pressed against the 
bob at the lower end of the chronometer. In this manner a fine line is 
traced on the tube, with which, when the instrument is w r ell regulated, the 
disjunctor marks ought to correspond. 
The indent made by the knife is a notch, clearly cut in the metal, the base 
of which is in a plane, perpendicular to the axis of the tube. It is the sec¬ 
tion of this plane with the tube which must be taken as the mark, and the 
index of the vernier must always be brought against the small step at this 
point when reading the height of the indent. 
The point of the vernier index is of the same form as the edge of the 
knife, and consequently fits accurately against the plane (or lower) side of 
the indent, so that there can be no uncertainty in the measurement. 
The instrument having been prepared, a disjunctor reading is taken; if 
the mark is exactly on the circle previously traced, and several repetitions 
show no variation, no alteration is necessary, and the experiments can be 
proceeded with at once. 
Should the mark, however, be above the circle, the space through which 
