Sandglass

Date of first hourglass is unknown. But they, like oil lampadnye appeared not earlier than glass. It is believed that in Western Europe on the sand clock learned only in the late Middle Ages, one of the oldest mention of them is the message of 1339, discovered in Paris. It contains instructions on the preparation of fine sand of the sifted powder black marble, prokipyachennogo in water and dried in the sun.

While the hourglass appeared in Europe much later, they quickly spread. This has been helped by their simplicity, reliability, price, and not least the opportunity to measure with the help of their time at any time of day and night. Their weakness was a relatively short time interval, which can be measured, not turning the device. Normal hours were calculated for half an hour or an hour, less often - 3 hours, and only in very rare cases, built huge hourglass to 12 hours of course. Not allowed to improve and connect several hourglass in a single unit.

Like fire, the hourglass never reaches exactly sunny. In addition, when durable goods they have changed their accuracy, because the grain of sand gradually fragmented into smaller and the hole in the middle of the diaphragm, on the contrary, gradually abrade and proliferate, so that the rate of sand passing through them is growing.