387 bce
Physics
The Greek philosopher Plato founds his school, the Academy, at Athens and begins his trilogy, Timaeus, Critias, and Hemocrates, finishing only Timaeus, in which he expounds his theory of four elements -- earth, water, air, and fire -- and hints at a fifth element, the ether. See also 450 bce Physics. (See biography.)
ToolsPlato is said to build an "alarm clock" to wake up his students. According to one account, it is a clepsydra that consists of a vessel slowly filling with water and in which floats a bowl with lead balls. When the bowl reaches the rim of the vessel, it topples over and the lead balls fall on a copper plate. Another reconstruction has Plato using two jars and a siphon. Water slowly empties through the night until it reaches the siphon, whereupon it swiftly is transported via the siphon to the other jar. Water rising in the other jar forces air through a whistle, sounding the alarm. See also 1380 bce Tools.






