An hourglass is a timekeeping device with two glass bulbs joined by a narrow neck, through which a measured amount of a granular substance, like sand, flows from the top bulb to the bottom due to gravity. The time it measures is determined by the quantity and quality of the sand, the width of the neck, and the size of the bulbs. Once the substance has fully transferred to the lower bulb, the hourglass can be turned over to measure time again.
Construction: It is typically made of two transparent glass bulbs connected by a narrow passage or "neck". A frame, often made of wood, may hold the glass bulbs together.
Material: The substance used is usually a fine, dry, granular material such as sand. Other materials like powdered eggshells or marble have also been used historically.
Function: It measures a specific