11. THE PLUMB-LINE APPARATUS.--This experiment, and many others, may be conveniently and accurately made with no other apparatus than a plumb line, and a device for sighting past it. In Figs. 6 and 7 there is shown a simple form of such apparatus, consisting essentially of a board which rests in a horizontal position upon the points of three screws that pass through it. This board carries a small box, to one side of which is nailed in vertical position another board 5 or 6 feet long to carry the plumb line. This consists of a wire or fish line with any heavy weight--e. g., a brick or flatiron--tied to its lower end and immersed in a vessel of water placed inside the box, so as to check any swinging motion of the weight. In the cover of the box is a small hole through which the wire passes, and by turning the screws in the baseboard the apparatus may be readily leveled, so that the wire shall swing freely in the center of the hole without touching the cover of the box. Guy wires, shown in the figure, are applied so as to stiffen the whole apparatus. A board with a screw eye at each end may be pivoted to the upright, as in Fig. 6, for measuring altitudes; or to the box, as in Fig. 7, for observing the time at which a star in its diurnal motion passes through the plane determined by the plumb line and the center of the screw eye through which the observer looks. The whole apparatus may be constructed by any person of ordinary mechanical skill at a very small cost, and it or something equivalent should be provided for every class beginning observational astronomy. To use the apparatus for the experiment of § 10, it should be leveled, and the board with the screw eyes, attached as in Fig. 7, should be turned until the observer, looking through the screw eye, sees Polaris exactly behind the wire. Use a bicycle lamp to illumine the wire by night. The apparatus is now adjusted, and the observer has only to wait for the stars which he desires to observe, and to note by his watch the time at which they pass behind the wire. It will be seen that the wire takes the place of the vertical edge of the building, and that the board with the screw eyes is introduced solely to keep the observer in the right place relative to the wire. 12. A SIDEREAL CLOCK.--Clocks are sometimes so made and regulated that they show always the same hour and minute when the stars come back to the same place, and such a timepiece is called a sidereal clock--i. e., a star-time clock. Would such a clock gain or lose in comparison with an ordinary watch? Could an ordinary watch be turned into a sidereal watch by moving the regulator? [Illustration: FIG. 8.--Photographing the circumpolar stars.--BARNARD.]