Mathematical recreations and essays . he radii of the circlesbeing about one inch. At one point of the band there is ahole; and when the ring is suspended from a fixed pointattached to the rims so that it hangs in a vertical plane con-taining the sun, the light from the sun shines through thishole and makes a bright speck on the opposite inner orconcave surface of the ring. On this surface the hours aremarked, and, if the ring is properly adjusted, the spot oflisfht will fall on the hour which indicates the solar time. The * An account of this sun-dial with a diagram was given in Knowledge, Lo
Mathematical recreations and essays . he radii of the circlesbeing about one inch. At one point of the band there is ahole; and when the ring is suspended from a fixed pointattached to the rims so that it hangs in a vertical plane con-taining the sun, the light from the sun shines through thishole and makes a bright speck on the opposite inner orconcave surface of the ring. On this surface the hours aremarked, and, if the ring is properly adjusted, the spot oflisfht will fall on the hour which indicates the solar time. The * An account of this sun-dial with a diagram was given in Knowledge, London,July 1, 1892, pp. 133, 134. t See Ozanam, 1803 edition, vol. in, p. 317; 1810 edition, p. 526. CH. XXIl] TIME AND ITS MEASUREMENT 453 adjustment for the time of year is made as follows. The rimsbetween which the band can slide are marked on their outeror convex side with the names of the months, and the bandcontaining the hole must be moved between the rims untilthe hole is opposite to that month for which the ring is For determining times near noon the instrument is reliable,but for other hours in the day it is accurate only if the timeof year is properly chosen, usually near one of the defect may be corrected by marking the hours on acurved brass band affixed to the concave surface of the possess two specimens of rings of this kind. These ringswere distributed widely. Of my two specimens, one was boughtin the Austrian Tyrol and the other in London. Astrolabesand sea-rings can be used as sun-rings. Clepsydras or water-clocks, and hour-glasses or sand-clocks,afford other means of measuring time. The time occupied bya given amount of some liquid or sand in running througha given orifice under the same conditions is always the same,and by noting the level of the liquid which has run throughthe orifice, or which remains to run through it, a measure oftime can be obtained. 454 TIME AND ITS MEASUREMENT [CH. XXII The burning of graduated candles
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