Everyday Mysteries: Fun Facts from the Library of Congress offers an explanation for how an hourglass measures time. Learn about the careful calibration of this age-old timepiece and what factors affect the accuracy of the sand clock. The website provides links to related websites that provide a brief history of the hourglass and sell custom hourglasses. The U.S Official Time website, which is also a link from this article, highlights exhibits that concern time (time keeping, calendars through the ages, daylight saving time, clockworks, and the quartz watch). For further reading, you can search the Library of Congress online database.