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A marine sandglass is a timepiece of simple design that is a relative of the common hourglass, a marine (nautical) instrument known since the 14th century (although reasonably presumed to be of very ancient use and origin). They were employed to measure the time at sea or on a given navigational course, in repeated measures of small time increments (e.g., 30 minutes). Used together with the chip log, smaller marine sandglasses were also used to measure the boat speed through the water in knots. Although vital to maritime navigation, marine sandglasses were not accurate measuring instruments for the passage of time; many design and environmental factors could affect the duration of sand's flow, and therefore its reported time. Their use continued through the early 19th century, when they were supplanted by reliable mechanical timepieces, and by other advances in marine navigation. Marine sandglasses were very popular on board ships, as they were the most dependable measurement of time while at sea. Unlike the clepsydra, the motion of the ship while sailing did not affect the hourglass. The fact that the hourglass also used granular materials instead of liquids gave it more accurate measurements, as the clepsydra was prone to get condensation inside it during temperature changes.〔Balmer, R. T. "The Operation of Sand Clocks and Their Medieval Development." Technology and Culture, Vol. 19, No. 4 (Oct., 1978), pp. 615-632 Balmer, R. T. "The Operation of Sand Clocks and Their Medieval Development." Technology and Culture, Vol. 19, No. 4 (Oct., 1978), pp. 615-632.〕 Seamen found that the hourglass was able to help them determine longitude, distance east or west from a certain point, with reasonable accuracy.〔 ==Design and use== Marine sandglasses originally consisted of two glass bottles one inverted above the other, connected by a smal tube, with the ends wrapped and so joined together. Over time, the later progress in the art of glassblowing allowed them to be made in a single piece. The marine glass was filled with sand or a suitable material such as finely ground eggshell, lead or tin chips (used to avoid humidity). This flowing material was chosen with two main objectives: to avoid the humidity and to absorb motion.. both required for shipboard use.〔〔 Placed in the upper half, the sand would flow slowly and steadily towards the lower half by the action of gravity, taking a certain time to empty (that was calibrated during their design and manufacture). Once the upper portion of the glass was empty, the glass could be turned to measure another time period. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Marine sandglass」の詳細全文を読む スポンサード リンク
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