372 
VELOCITY AND PRESSURE INSTRUMENTS. 
being made, so as to include the latest improvements in the instrument and 
in the method of employing it. 
The principle of action consists in registering, by means of electric 
currents upon a recording surface travelling at a uniform and very high 
speed, the precise instant at which a shot passes certain defined points in 
the bore. 
It consists of two portions—firstly, the mechanical arrangement for 
obtaining the necessary speed and keeping that speed uniform; secondly, 
the electrical recording arrangement. (See Plate Y.) 
The first part of the instrument consists of a series of thin metal discs, 
AA (Plate Y., Pig. 1), each 36 ins. in circumference, fixed at intervals upon 
a horizontal shaft, SS, which is driven at a high speed by a heavy descending 
weight acting on a chain, B —arranged according to a plan originally pro¬ 
posed by Huyghens, through a train of gearing, F, multiplying 200 times. 
The driving weight is, during the experiment, continually wound up by 
means of the handle, T. 
If the requisite speed of rotation were got up by the action of the falling 
weight alone, a considerable waste of time would ensue. To obviate this 
inconvenience, the required velocity can be obtained with great rapidity by 
means of the handle, C. 
The precise rate of the discs is ascertained by means of the stop-clock, B, 
which can, at pleasure, be connected or disconnected with the revolving 
shaft, E (Fig. 1), and the time of making any number of revolutions of this 
shaft can be recorded with accuracy to the one-tenth part of a second. 
The speed usually attained in working the instrument is about 1100 ins. 
per second linear velocity at the circumference of the revolving discs, so that 
each inch travelled at that speed represents somewhat less than the one- 
thousandth part of a second; and as the inch is subdivided by the vernier, 
V, into a thousand parts, a linear representation, at the circumference, is 
thus obtained of intervals of time as minute as the one-millionth part of a 
second. As a small variation in speed would affect the relation between the 
several records obtained, the uniformity of rotation is ascertained on each 
occasion of experiment by three observations—one immediately before, one 
during, and one immediately after the experiment—the mean of the three 
observations being taken for the average speed. 
With a little practice, there is no difficulty in arranging the instrument so 
that the discs may rotate either uniformly, or at a rate very slowly increasing 
or decreasing. 
The modus operandi is as follows :—When the requisite speed of rotation 
has been attained, the stop-clock is connected with the shaft, E, and the time 
occupied by the wheel, F, in making five revolutions—that is, the time 
occupied by the discs, A, in making 1000 revolutions—is recorded. 
After the wheel, F, lias made one revolution, unconnected with the clock, 
the time of making 1000 revolutions is again observed, and in the middle of 
this observation the gun is fired. 
After the wheel, F\ has again made one revolution, the time of 1000 revo¬ 
lutions is once more recorded, and the instrument is stopped. 
An idea of the degree of uniformity realised, when the instrument is in 
working order, will be furnished by the observations of speed of the instru¬ 
ment for six consecutive rounds, which are given on the opposite page. 
