The hourglasses are water clocks.

  that was most likely invented in Pharaonic Egypt. The oldest surviving hourglass comes from Karnak, from the time of Amenhotep III, 18th dynasty. Carved from a block of calcite in the shape of an inverted truncated cone. The container was filled with water and this was evacuated through a small hole located above the base of the same. In the inner wall, there were holes arranged in scales (1 for each month of the year) so that when the water level fell from one mark to another it meant that an hour had passed. In Egypt, the day was split equally into twelve hours for the night and as many for the day, logically the value of each hour varied according to the season of the year.