VELOCITY AND PRESSURE INSTRUMENTS. 
359 
This arrangement is no doubt very convenient in cases where it is required 
to move the instrument constantly, and to set it up in different positions. 
Tor proving powder, or in similar cases, where the instrument is stationary, 
it is advisable to have recourse to a more permanent arrangement. The 
plan adopted at Waltham Abbey is to form a stand by driving four piles 9 ft. 
into the ground, on the top of which is placed a solid oak block, 15i ins. 
square and 2\ ins. thick, so as to form a permanent stand 3 ft. 6 ins. high. 
The piles are out of contact with the floor or walls.of the room, to prevent 
vibration being communicated to the instrument. 
The triangular piece which supports the trigger and the column is fastened 
to a heavy cast-iron base by the three screws supplied with the instrument. 
This base is 13 ins. square and 1 in. thick, and is supported on three 
milled-headed levelling screws, which work in brass Ws let into the oak block. 
The piles are cased with deal to form a box, and a recess is cut in the iron 
base and the oak top to allow the chronometer to fall into a vertical trough 
made to receive it. 
This trough, as well as the edge of the recess in the iron base, is lined 
with leather, and it is also padded at the bottom, to prevent the chronometer 
from rebounding, and the point from being injured by striking against the 
iron stand. The instrument is permanently fixed to the stand, and is 
covered, when not in use, by a wooden case, to protect it from injury, a 
small saucer containing calcined chloride of calcium being used to absorb 
the moisture under the case. 
The electro-magnets are fixed in position by passing the screwed stems 
through the column and fastening them with milled-headed nuts. Two 
zinc tubes, or registers, are placed on the chronometer; to put on the 
small one, the bob at the lower end must first be unscrewed. The 
tubes should be pressed slightly out of shape before being put on, to cause 
them to fit tightly on the rod, and not to shift too easily. Care must be 
taken during the operation that the bottom of the tube is resting against the 
bob. 
At Waltham Abbey the disjunctor is fixed by wooden cleats to the side of 
the shed on the right of the instrument, so as to be in a convenient position 
for the operator when standing opposite the chronograph. 
The connexions with the battery and the screens having been established, 
and the currents found to pass correctly, and to be of sufficient strength, the 
next step is to adjust and regulate the instrument. 
This consists of three operations, viz.:— 
(1) Levelling the instrument. 
(2) Regulating the power of the electro-magnets. 
(3) Regulating the height of the disjunctor reading. 
(1) Levelling the Instrument .—Tor this purpose the chronometer is used. 
After having cocked the trigger, suspend the chronometer to its electro¬ 
magnet, and bring it into its proper position by means of the levelling 
screws. 
In levelling from front to rear, see that the inclined plane on the bob, on 
the side opposite the number, rests very lightly against the projecting edge, 
cc, (Plate II., Tig. 1), of the triangular base. 
To level laterally, the right face of the bob is brought exactly in line with 
the salient angle, dd, formed by the projection above referred to. In this 
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