It is at once apparent that by the introduction of this application of electricity, astronomy has been an enormous gainer; but so far we have simply given a description of one instrument which has been suggested for that purpose. A few words may be said on other forms. In the instrument used in the Royal Observatory at Greenwich the rotation of the roller is kept uniform, as we have seen, by a conical pendulum; but there are other methods of attaining this end—there is the fly-wheel and fan, similar to the arrangement for regulating the striking part of a clock; there is the governor used for the steam-engine, and others which give a fairly regular motion—for the motion need not be absolutely uniform, because the dots, which form the points from which to measure, are made by the standard clock. The particular instant at which each minute occurs may be recorded in another way. The two steel springs above described may be pressed together, not by a pin in the crutch, but by cogs on a wheel attached to the spindle of the escape-wheel of the clock (see Fig. 128); and then all we have to do to stop the transmission of a current at the sixtieth second is to remove one of the cogs. [Illustration: FIG. 128.—Wheel of the sidereal clock, and arrangement for making contact at each second. ] Another simple method for transmitting seconds’ currents has also been occasionally tried. A wire runs down the whole length of the pendulum, and ends in a projection of such a length that it swings through a small globule of mercury in a cup below it, the pendulum being connected with one wire from the chronograph and the mercury with the other; thus there will be a making and breaking of contact each time the point of the pendulum swings through the mercury. It is uncertain which method is the better; one would prefer that which, under any circumstances, could disturb the pendulum least: but as to which this is opinions differ. We have now described the _modus operandi_ of making time observations with the transit instrument, the final result of which is that the time shown by the sidereal clock corresponds with the right ascension of the “clock stars” as they transit the central wire. The great use, as we have already stated, made of the sidereal clock thus kept right by the stars is to correct the mean-time clock with a view of supplying mean solar time to the outside world. As the sidereal clock is regulated by the stars, it can be corrected by them at any time by the clock stars given in the “Nautical Almanac,” whose time of passing the meridian is calculated beforehand much more accurately than a mean-time clock could be corrected by the sun; we therefore correct our mean clock by the sidereal, the two agreeing at the vernal equinox, when the sun is in the first point of Aries, and the sidereal clock gaining about 3_m._ 56_s._ each day until it has gained a whole day, and agrees again at the next vernal equinox. [Illustration: