what is sand clock

An hourglass, also known as a sand clock or sand timer, is a device used to measure the passage of time. It consists of two glass bulbs connected vertically by a narrow neck that allows a regulated flow of sand from the upper bulb to the lower one by gravity. The sand used in an hourglass must have a rate of flow that does not fluctuate, and it must be fine, dry, and consistently formed so it can flow smoothly. The accuracy of an hourglass depends on several factors, including the amount or volume of sand used, the size and angle of the glass bulbs, the width of the neck, and a tight seal so no moisture can get into the chambers. While hourglasses are no longer widely used for keeping time, some institutions still maintain them, and they are sometimes included with board games such as Pictionary and Boggle that place time constraints on rounds of play.