CERTAIN INDICATING AND RECORDING APPARATUS. IBl 
or by causing the circuit of an electric current to be made and 
broken with a frequency proportional to the velocity to be 
measured. The use of a fluid column need not be discussed, it 
has obvious disadvantages, and, at any rate, has not hitherto 
been successfully employed where accuracy is necessary and the 
distance considerable. The use of intermittent action, for 
maintaining or regulating the continuous motion of a revolving 
piece in correspondence with it, is a problem w^hich offers great 
difficulties in its solution, and, indeed, to maintain exact cor- 
respondence appears to be impossible. 
The first method, viz., conjoint observation of time and 
space, is employed with surface and subsurface floats on the 
measured mile, with ordinary anemometers and current meters, 
or wffien the body is revolving, with revolution counters. It has 
great practical advantages when an immediate observation is not 
necessary, and it can clearly be employed at a distance from the 
body in motion. There are, however, two obvious defects. The 
first of these is the time required for the observation ; the 
second is that only the average velocity during the time of an 
observation is given, the expression for this being 
Average veL = A — — 
time t 
The author has endeavoured to solve the problem of devising 
an instrument communicated with by a series of intermittent 
effects, but which should indicate continuously the velocity of 
the moving body. That is, to shew on a dial v = — where As 
A t 
and A t could be reduced to any required amount by sufficient 
frequency of the effects. 
The following is a theoretical solution : — 
Let A [Fig. 1) be a disk upon which work, by frictional 
contact, two rollers, B and C ; one of the rollers, By really forms 
a nut, working freely upon its axis, E, as a screw. This axis is 
L 
