How an Hour Glass Works
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.
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.
Generally the hour glass is capped by a piece of wood at either end. These are held tight to the glass by screws reaching through the end-pieces of would and tightening down on three spindles that clamp the wood tight to the glass. In manufacture, the amount of sand necessary for each slightly different opening between the two bulbs of glass could be gauged before the end pieces were put on. Also the glasses could be adjusted by removing one of the wooden ends and either adding more sand or taking some away.
The hour glass has many advantages that recommended it to the ancients as well as Medieval mind. First it was fairly easy to make. Nearly any glass blower could construct one. Second, it did not have mechanical parts that would wear or need repair. It was and is a closed system that runs with regularity despite the climate or barometric pressure. It can be made small and portable. Unlike a sundial it could be used at night.
Invented in ancient Egypt, probably Alexandria in about 300 B.C. the use of the hour glass spread throughout the ancient world. It was certainly known of in Ancient Athens. They served a purpose for rough time keeping throughout the middle ages and even saw use in churches in England, where they were used to limit the loquaciousness of fervent preacher's sermons to under one hour.
The hour glass was used by the navies of the world until regular chronometers began to supplant them in the early 1800s. The entire ship's company gauged its day's activities by the turn of the hour glass and a subsequent ringing of bells. A small, 1/2 minute sand-timer was used at sea to measure speed through the water. In this case, a knotted rope (with the knots spaced at equal intervals) was thrown overboard as the ship moved through the water. Knots were spaced so that each knot that passed through the fingers of a midshipman or seaman during the time the sand would run measured one nautical mile per hour. As this could only measure speed through the water, it did not measure the ship's actual speed especially in a current.
Does tapping an hour glass make it run any faster? Not so that you would notice, however, if there are any grains left sticking to the sides of the glass at the end, tapping will knock them loose.
The hour glass has found its way into the modern world as a symbol. The physical figure of a woman is often described as an hour glass. This term is generally used as a compliment. We also often see an hour glass used in the place of cursors on computer screens to tell the user to wait while the computer is working.
Although the hour glass and the sand timer have their uses, they are really only adequate for very short term time measurement as they require someone to observe and turn them at the appropriate moment. They are also not good for tracking gradations in time, such as marking off minutes or seconds. Most people would not try to time a horse race using an hour glass. More precision with less trouble can be achieved with mechanical clocks.
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