MODES OF MEASURING MINUTE INTERVALS OF TIME. 115 
movement, the times may be deduced. Nothing offers itself so readily for this purpose as 
the motion of the pendulum ofa time piece. The are of movement corresponding to one se- 
cond is sufficiently uniform in its amplitude, and is extensive enough to admit of being di- 
vided into spaces affording probably as minute and as correct measures of the fractions of a 
second as our physiological constitution will admit of our taking into account in practical 
operations. To render it available for this purpose, a metallic are concentric with the arc 
of vibration is to be placed exactly under the pendulum, and resting on glass, so as to be 
in a state of electric insulation. A fine point in the axis of the pendulum vibrating above 
this metallic arc, will communicate or receive slight galvanic discharges, marking the pe- 
riod during the vibration at which, by means of the common apparatus recommended for 
such purposes, a galvanic contact is effected. If, between the metallic are and the pen- 
dulum point, there be interposed a film or riband of paper chemically prepared to receive 
and retain a mark from the passing galvanic influence, the position of this mark will in- 
dicate the instant of the signal, measured from the beginning of the vibration. The paper 
may be ruled with parallel lines transverse to the movement of the vibrating point, so as 
either to mark equal subdivisions of the arc, from which the times may be deduced, or so as 
at once to present intervals corresponding to equal divisions of time. A time-movement of 
any character drawing this paper along, will secure the proper record of the signals made 
at the occurrence of the observed phenomena. It may be a subject of inquiry whether 
the galvanic influence passing between the metallic arc and the vibrating point, would 
not tend to disturb the rate of the clock. The same objection, if sustainable, will affect 
all time-movements on which signals are impressed by galvanic influence. The extent to 
which this tends to interfere with absolute accuracy, may be the subject of interesting ex- 
periment, if the case requires it. In the case of the pendulum it will be advisable to ex- 
tend the metallic are to some distance beyond the amplitude of the vibration. With this 
precaution to compensate for irregularity of force at the ends of the arc, if such should be 
apprehended, and having the electric tension of a low rate, it is not probable that its in- 
troduction, in giving a signal, would have any sensible effect in the rate of the clock. The 
effect, if capable of being recognised, would be estimated as a correction. 
It may be worth inquiry, whether celestial phenomena may not, in certain cases, be 
made to give of themselves permanent signals of the time of this occurrence, so as to re- 
duce such observations to mere linear measurements, or to coincidences with measures 
already made and marked. This would be accomplished if a star’s pencil of rays could 
trace the star’s path across the field of view in a permanent form upon a surface. Photo- 
eraphy has reached such a condition as apparently to bring this within our power. The 
preparation composed of albumen, iodide of iron, alcohol and acetic acid, has been found 
