We can easily see at once that it would not do to have an ordinary clock regulated by a pendulum for driving the telescope, it would be driven by fits and starts, which would make the object viewed jump in the field at each tick of the pendulum. The most simple clock is therefore one in which the conical pendulum is used in the form of the governor of a steam-engine, so that when the balls A A, Fig. 146, fly up by reason of the clock driving too fast, they rub against a ring, B, or something else that reduces their velocity. [Illustration: FIG. 146.—Clock Governor. ] [Illustration: FIG. 147.—Bond’s Spring Governor. ] There is another form made by Alvan Clarke, in which a pendulum regulates the clock, but not quite in the ordinary way. The drawing will perhaps make it clear: A A, Fig. 147, is one of the wheels of the clock-train driving a small weighted fan, B, which is regulated so as to allow the clock to drive a little too fast. Now let us see how the pendulum regulates it. On the axis of A A is placed an arm, C, which is of such a length that it catches against the studs S S, and is stopped until the pendulum, P, swings up against one of the studs, R, which moves the piece D, like a pendulum about its spring at E, until the stud, S, is sufficiently removed to let the arm, C, pass, so that the clock is under perfect control. If, however, the arm C were fixed rigidly to the axis of the wheel A A, there would be a jerk every time C touched one of the studs. The wheel is therefore attached to the axis through the medium of a spring, F, so that when the arm is stopped the wheel goes on, but has its velocity retarded by the pressure of the spring. The pendulum is kept going in the following manner:—There is a pin fixed to the axis on the same side of the centre as C, which, as the arm approaches either stud S, raises the piece D, but not sufficiently to liberate the arm; the pendulum has then only a very little work to do to raise D and disengage the arm, C; but as soon as it is free it starts off with a jerk, due to the tension of the spring on the axis, and leaves D by means of its stud, R, to exert its full force on the pendulum and accelerate its return stroke, so that the pendulum is kept in motion by the regulating arrangement itself. The late Mr. Cooke of York constructed a very accurately-going driving clock. This differs in important particulars from Bond’s form, though the control of the pendulum is retained. The following extracts from a description of it will show the principal points in its construction:—