the case may be, and the mechanism continues to be in good working order.
COMPENSATING PENDULUMS.
Metal expands when heated; therefore a steel pendulum which is of the exact length to govern a clock correctly at a temperature of 60° would become too long at 80°, and slow the clock, and too short at 40°, and cause it to gain. In common clocks the pendulum rod is often made of wood, which maintains an almost constant length at all ordinary temperatures. But for very accurate clocks something more efficient is required. Graham, the partner of Thomas Tompion, took advantage of the fact that different kinds of metal have different ratios of expansion to produce a self-compensating pendulum on the principle illustrated by Fig. 203. He used steel for the rod, and formed the bob, or weighted end, of a glass jar containing mercury held in a stirrup; the mercury being of such a height that, as the pendulum rod lengthened with a rise of temperature, the mercury expanded upwards sufficiently to keep the distance between the point of suspension and the centre of gravity of the bob always the same. With a fall of temperature the rod shortened, while the mercury sank in the jar. This device has not been improved upon, and is still used in observatories and other places where timekeepers of extreme precision are required. The milled nut S in Fig. 203 is fitted at the end of the pendulum rod to permit the exact adjustment of the pendulum's length.
For watches, chronometers, and small clocks
THE SPRING BALANCE
takes the place of the pendulum. We still have an escape-wheel with teeth of a suitable shape to give impulses to the controlling agent. There are two forms of spring escapement,