360 
VELOCITY AND PRESSURE INSTRUMENTS. 
position the left face of the boh is a short .distance from the screw, E ; the 
edge of the knife is opposite and slightly behind the zinc tube; and when 
the chronometer falls, the projecting ring c (Fig. 1), passes clear of the 
knife-edge. To test whether this is the case, break the circuit, by means of 
the disjunctor, and notice whether there is any friction, or if anything catches 
during the fall. 
To ascertain whether the chronometer is properly levelled from front to 
rear, see that the inclined plane on the bob rests along its whole length 
against the projecting edge, then remove it sideways out of the vertical, by 
pushing the bob against the screw, E, when it will return into its original 
position, if properly levelled. 
It is of great importance that the instrument should be accurately level, 
otherwise the distance of the chronometer from the knife-edge will vary 
during its fall, and the distance the knife has to travel before reaching the 
zinc will not always be the same; in other words, t'" will not be a constant. 
The levelling being completed, and the registrar suspended to its electro¬ 
magnet, the inclined plane on the bob, on the side opposite the number, 
should rest very lightly against the edge of the arm, K. This arm is fitted in 
a bracket, and its position can be altered by means of an adjusting screw. 
This adjustment need only be performed once for all ordinary positions of the 
instrument when used in taking velocities. 
(2) Regulating the Electro-Magnets .—This is done (as with the Navez- 
Leurs instrument) by withdrawing the core of the magnets until they are only 
capable of just supporting the rods. It is always an advantage to work with 
weak magnets, as the variation in the time required to demagnetise them 
need not be taken into account; should, however, their power be insufficient, 
the operator will experience some difficulty in suspending the rods. 
The following method has been adopted for making the electro-magnets of 
just sufficient power :— 
The chronometer, with its zinc tubes, is increased in weight by means of a 
brass tube one-tenth the weight of the chronometer, which is slipped over the 
upper zinc. It is then suspended to the magnet, and the core gradually 
withdrawn until the power is insufficient to support the weight, when it falls. 
The core must be turned slowly and gently, so as not to free the rod by any 
jar or vibration, and it is well to ascertain that the rod has not been shaken 
down by offering it to the magnet a second time before removing the extra 
weight. The extra weight is then removed, and the chronometer can be 
suspended without difficulty. 
The other magnet is regulated in the same manner, a smaller brass tube 
being supplied for increasing the weight of the registrar. 
In order to suspend the chronometer to its electro-magnet without diffi¬ 
culty, the following method should be adopted :—Hold it lightly in the left 
hand at the centre, the fingers open and towards the body; allow the bob to 
rest upon the second joint of the first finger of the right hand, the hand 
being half open, the palm vertical and turned towards the body, and the 
fingers together ; the chronometer is thus held in a vertical position, the 
numbered face of the bob being turned towards the operator. Bring it, in 
this position, to the electro-magnet, by placing the exterior surfaces of the 
two cones in contact, and, as soon as attraction is perceived, let go with the 
left hand, still keeping the fingers near to catch the chronometer should it 
fall. Then slowly lower the right hand, so that the two cones, sliding over 
