The hourglass, also known as the sand clock, is a device for measuring time. The western hourglass consists of two glass balls and a narrow connecting pipe. The time is measured by the time required for the sand filled with the upper glass ball to flow through the narrow pipe into the bottom glass ball. Once all the sand has flowed to the bottom glass ball, the hourglass can be reversed to measure the time. The general hourglass has a nominal running time of 1 hour.