The origin of the hourglass is uncertain. It is believed that the Roman armies used them during the night; It was also said that they were invented by a French monk at the end of 8th century. At that time, Charlemagne, the king of the Francs, had one so large that it only needed to be turned over every 12 hours. Hourglasses were commonly used to establish working shifts during sea voyages . Obscuriosity offers several models.