The pair of segments σ, γ, and δ, ε, are connected to η θ, the other pair of segments to η′ θ′. Now suppose the discs turning with the arrows: If K rests on one of the insulated points when the pendulum throws the battery C Z into circuit nothing happens. If the disc is gaining on the pendulum, K, instead of resting on γ as shown will contact with segment γ, σ, and actuate a relay via V, exciting the appropriate brake magnet. [Illustration: FIG. 137.—Sir David Gill’s Electric Control.] If the disc is losing, K contacts with segment γ, δ, and current will pass via η′θ′ and U to a relay that operates the other brake magnet and lets the clock accelerate. A fourth disc (not shown) on the same spindle is entirely insulated on its edge except at points corresponding to γ and ε, and with a contact spring like K. If the disc is neither gaining nor losing when the pendulum makes contact, current flows via this fourth disc and sets the relay on the mid-point ready to act when needed. This clock is the prototype of divers electrically-braked driving clocks with pendulum control, and proved beautifully precise in action, like various kindred devices constructed since, though the whole genus is somewhat expensive and intricate. The modern tendency in driving apparatus for telescopes, particularly large instruments, is to utilize an electric motor for the source of power, using a clock mechanism merely for the purpose of accurately regulating the rate of the motor. We thus have the driving clock in its simplest form as a purely mechanical device worked by a sensitive fly-ball governor. The next important type is that in which the clock drive is precisely regulated by a pendulum clock, the necessary governing power being applied electrically as in Fig. 137 or sometimes mechanically.