The reason that the table given here is convenient for setting clocks to mean time is that a minute is as close as a dial can be read, but if you wish for greater accuracy, then the almanac, which gives the "equation of time" to a second for each day, will be better. The reason that these noon-mark dials are better than ordinary commercial dials is that they are larger, and still further, noon is the only time that any dial is accurate to sun time. This is because the sun's rays are "refracted" in a variable manner by our atmosphere, but at noon this refraction takes place on a north and south line, and as that is our noon-mark line the dial reads correctly. So, for setting clocks, the corner of your house is far ahead of the most pretentious and expensive dial. In Fig. 4 is shown a modern horizontal dial without the usual confusing "ornamentation," and in Fig. 5 it is shown set up on the latitude of New York City for which it is calculated. This shows clearly why the edge FG of the style which casts the shadow must be parallel to the earth's axis and why a horizontal dial must be made for the latitude of the place where it is set up. Figure 6 is the same dial only the lines are laid out on a square dial plate, and it will give your young scientific readers a hint of how to set up a dial in the garden. In setting up a horizontal dial, consider only noon and set the style, or 12 o'clock line, north and south as described above for noon-mark dials. [Illustration: Fig. 6--Modern Sundial Set Up in Garden] A whole issue of Popular Mechanics could be filled on the subject of dials and even then only give a general outline. Astronomy, geography, geometry, mathematics, mechanics, as well as architecture and art, come in to make "dialing" a most charming scientific and intellectual avocation. During the night and also in cloudy weather the sundial was useless and we read that the priests of the temples and monks of more modern times "went out to observe the stars" to make a guess at the time of night. The most prominent type after the shadow devices was the "water clock" or "clepsydra," but many other methods were used, such as candles, oil lamps and in comparatively late times, the sand glass. The fundamental principle of all water clocks is the escape of water from a vessel through a small hole. It is evident that such a vessel would empty itself each time it is filled in very nearly the same time. The reverse of this has been used as shown in Fig. 7, which represents the "time-boy" of India. He sits in front of a large vessel of water and floats a bronze cup having a small hole in its bottom in this large vessel, and the leakage gradually lowers this cup till it sinks, after which he fishes it up and strikes one or more blows on it as a gong. This he continues and a rude division of time is obtained,--while he keeps awake! [Illustration: Fig. 7--"Time-Boy" of India]