@we86ll
Active 3 년 전
http://www.skyringecraft.com/sand-timer-hourglass/
How does an hourglass measure time?
The sand timer hourglass is sometimes referred to as a sand clock or a sandglass. Like other timepieces, it needs to be carefully calibrated. The hourglass maker must test the instrument and fine tune it to measure the correct length of time.
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