2 
MR. H. M'LEOD AND LIEUTENANT G. S. CLARKE ON THE 
lum worked by electricity and connected with an electric clock. The method of 
compensation was copied from that of a pendulum exhibited in the Loan Collection of 
Scientific Apparatus, as a model of the one belonging to the standard clock of the 
Royal Observatory at Greenwich. Originally, the impulse was given by an electro¬ 
magnet placed under the bob, the contact being made by a small trailing piece of steel, 
which, catching in a notch in a piece of steel below, depressed a spring at every alternate 
second. This method, which is frequently employed in small clocks, is generally arranged 
to give a powerful impulse to the pendulum, and this so much increases the arc of 
vibration that the trailing piece passes over the notch without again engaging in it until 
the arc has sufficiently diminished. Although such clocks are accurate enough for 
ordinary purposes, it is obvious that the method would be inadmissible in cases where 
the period of all vibrations must be the same. To ensure uniformity in this respect 
the battery power was diminished until the contact was made at each vibration, but 
after many trials the method was abandoned, for it was found that the friction between 
the steel trailing piece and the notched piece below was not constant, and, besides, 
the pendulum was often found to be swinging in an elliptic arc. 
An endeavour was next made to work the pendulum by a driver depending on the 
action of gravity alone, the electric current being employed merely to lift the driving 
lever, so that the impulse given to the pendulum was entirely independent of the 
power of the battery. It will be unnecessary to detail the various experiments that 
were made with more or less success. The following is a description of the form of 
driving arrangement ultimately adopted. 
On each side of the supporting spring of the pendulum is pivoted a branched lever 
a, b, Plate 1, fig. 1. At the lower end of each lever is a platinum pin which comes 
in contact with the rod of the pendulum as in many forms of gravity escapement. The 
distance of fall of each lever is regulated by means of screws, c c. The lever arm, a, is 
so adjusted that the pendulum rod touches the pin at the end of its swing, thus making 
an electric contact with a platinum collar placed on the pendulum rod, and causing 
the current to flow from the battery through the electro-magnet d, attracting the 
armature and raising the lever e. [This arrangement is not shown in the figure, which 
represents a slight modification to be described presently.] A pin, p, projecting from 
the lever e, lifts the driving lever b. On the return swing of the pendulum to the 
right the contact with a is broken and the lever e falls, leaving b suspended by a small 
steel catch, f When the pendulum rod touches the platinum pin on the end of b, a 
current traverses the releasing magnet g, withdrawing the catch, and allowing the 
driving lever b to fall with the pendulum and thus to give the requisite impulse. Many 
accurate determinations were made with the contacts as above described, but it was 
found that the roughening of the pin and platinum collar by the spark produced 
variable friction; this led to the adoption of the arrangement shown in the figure, 
which consists in attaching (by an ivory pin, h h) a thin piece of watch spring to 
each lever a and b, so that the electric contacts are made between the platinum pin 
