The form of governor most usually employed will be seen in figures previously given. The governor raises a plate and thus becomes a frictional governor, by which all overplus of power is used up in frictions, or by that doubling the driving power no, or only a small, difference should be brought about in the rate. Other forms of driving clocks or governors invented by Foucault and Yvon Villarceau are now being largely employed. In them the rapid motion of a fan and other devices are introduced. A driving clock adjusted to sidereal time requires adjustments for observations of the sun and moon. This (as at _z_ in Fig. 144) is sometimes done once for all by differential gearing thrown into action by levers when required. Mr. Grubb has lately made a notable improvement upon the usual form by controlling the motion of the governor by a sidereal clock and an electric current. There are various methods of attaching the clock to the polar axis. One is to make the clock turn a tangent screw, gearing into a screw-wheel on the axis of the telescope, which can be thrown in and out of gear for moving the telescope rapidly in right ascension. Another method is to have a segment of a circle on the polar axis which can be clamped or unclamped at pleasure by means of a screw attached to it. A strip of metal is attached to each end of the segment and is wound round a drum turned by the clock, so that the two are geared together just as wheels are geared by an endless strap passing round them. This arrangement gives a remarkably even motion to the telescope. When the strap is wound up to the end of the segment, which is done in about two or three hours’ work, the drum is thrown out of gear and the arc pushed back to its starting-place again. _THE LAMP._ [Illustration: FIG. 149.—Illuminating lamp for equatorial. ]