The distance for the line marked V from O′ is just the same as the distance from O to VII. Similarly, IV corresponds to VIII, III to IX, II to X, and I to XI. The number XII is marked at MM′ as shown. If you desire to add lines (not shown in Fig. 3 to avoid confusion) for hours earlier than six in the morning, it is merely necessary to mark off a distance on the line KO, below the point O, and equal to the distance from O to VII. This will give the point where the 5 A.M. shadow line drawn from N cuts the line KO. A corresponding line for 7 P.M. can be drawn from N′ on the other side of the figure. After you have marked out the dial very carefully, you must fasten the three-cornered shadow-piece to it in such a way that the whole instrument will look like Fig. 1. The edge _ac_ (Fig. 2) goes on NM (Fig. 3). The point _a_ (Fig. 2) must come exactly on N (Fig. 3); and as the lines NM (Fig. 3) and N′M′ (Fig. 3) have been made just the right distance apart to fit the thickness of the three-cornered piece _abc_ (Fig. 2), everything will go together just right. The point _c_ (Fig. 2) will not quite reach to M (Fig. 3), but will be on the line NM (Fig. 3) at a distance of three inches from M. The two pieces of wood will be fastened together with three screws going through the bottom-board ABCD (Figs. 1 and 3) and into the edge _ac_ (Fig. 2) of the three-cornered piece. The whole instrument will then look something like Fig. 1. After you have got your sun-dial put together, you need only set it in the sun in a level place, on a piazza or window-sill, and turn it round until it tells the right time by the shadow. You can get your local time from a watch near enough for setting up the dial. Once the dial is set right you can screw it down or mark its position, and it will continue to give correct solar time every day in the year. If you wish to adjust the dial very closely, you must go out some fine day and note the error of the dial by a watch at about ten in the morning, and at noon, and again at about two in the afternoon. If the error is the same each time, the dial is rightly set. If not, you must try, by turning the dial slightly, to get it so placed that your three errors will be nearly the same. When you have got them as nearly alike as you can, the dial will be sufficiently near right. The solar or dial time may, however, differ somewhat from ordinary watch time, but the difference will never be great enough to matter, when we remember that sun-dials are only rough timekeepers after all, and useful principally for amusement. FOOTNOTE: [A] This chapter is especially intended for boys and girls and others who like to make things with carpenters' tools. PHOTOGRAPHY IN ASTRONOMY