“An hourglass is a piece of blown glass that is pinched in the middle. Sand is sealed inside the glass. When the glass is tipped over the sand pours slowly through the pinched center at a constant rate until all of the sand in the top flows to the bottom which is of equal size and shape. The turning of the hour glass sends the grains again from top to bottom at the same rate, thus the timer or glass will measure time equally whichever side is turned up. If the sand is measured, one can use the glass to measure the passage of time.
hour glass image.
An hour glass can be somewhat precise because the pressure at the base of a pile of sand does not increase as the height of the sand increases. Thus, sand grains will trickle through the narrow opening of an hourglass at a constant rate no matter how many are above, pushing down. This means the hour glass can be graduated to signify the passing of minutes as well as hours.”
How Clocks Work