CERTAIN INDICATING AND RECORDING APPARATUS. 133 
intermittent, as it would only mean that there would be a 
continual oscillation of the roller, B, one way or the other to an 
extent which might be indefinitely reduced. It must, of course, 
be remembered that the roller, B, does not move instantaneously 
to its new position, but, on the other hand, it does so with a 
speed which varies in the direct ratio of the change of velocity 
of the moving body.^' 
To put these principles into a practical shape, an instrument 
was devised, in which, for convenience, the axis of the disk is 
driven directly by the clock. Two rollers are used, but each 
worked on a separate screw. They are arranged to measure at 
once the velocities of two different bodies, and not only indicate 
them both by two separate hands on a dial, but also record them 
upon a drum. The proposed object of this particular instrument 
was to record by suitable arrangements the speed of the engines 
of a ship, and at the same time its velocity through the water. 
Figs. 5, 6, 7, and 8 are different views of the instrument. The 
disk A {Figs. 5 and 6) is driven by an electro-magnet, M, and 
ratchet wheel, W, the action of which is controlled by the clock, 
K, by means of a special contact maker, Z. Two rollers, B 
and C, shewm together in their central position in Fig. 5, work 
upon the screws E and D, the lower one of which is hollow and 
is driven by the upper ratchet wheel, Y, by an electro-magnet, 
N. The upper screw is reduced so as to pass through the 
hollow one, and is driven in a similar way by the lower ratchet 
wheel, X. From what has been previously said the action of 
the instrument will be easily understood. The rollers, B and C, 
act respectively upon two racks, 0 and P, and, by their means, 
upon the pinions of the indicating hands and upon the recording 
drum. 
* The author afterwards ascertained that a suggestion was 
previously made of this mode of measuring velocity by a cor- 
respondent in Engineering, -vol. xx., p. 314. 
