Measurement
The Hourglass
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Grains trickle at a steady rate from the top chamber to the bottom through a narrow neck, marking off a fixed span of time no matter how the hourglass is tilted or handled.
Turn the hourglass over and time it with a clock. Does it take exactly the same amount of time on every run?
Additional Information
The flow rate stays remarkably constant even as the grain pile height changes, because it’s set by the narrow neck opening, not by the weight of sand above it.
Hourglasses were once essential at sea, since a swinging ship could throw off pendulum clocks but not a sealed hourglass.
Sand isn’t the only option — early versions used crushed eggshell or powdered marble, chosen for flowing at a steady, predictable rate.