How does an hourglass measure time?

An hourglass has two glass bulbs joined by a narrow opening. When it is turned over, the sand flows from the top bulb to the bottom one, and the time it takes depends on the amount of sand, the size of the grains and the width of the neck. Unlike water, which drains faster when there is more of it pushing down, sand flows at an almost constant rate whatever the height of the pile above, because the grains support each other against the sides of the glass. This makes an hourglass a surprisingly reliable timer. Hourglasses were widely used in Europe from the 14th century, including on ships, where they helped sailors measure time and estimate their speed.