THERMOPYLAE, a Greek pass leading from Locris into Thessaly between Mount Oeta and the Maliac Gulf ; chiefly famous for the heroic defence by the Spartan king, Leonidas, with 30o soldiers against the Persian army of Xerxes advancing upon Greece in 480. (See LEONIDAS.) Two other battles here are famous. In 279 B.C. Brennus and the Gauls were checked for several months by a Greek army under the Athenian Calippus; and in 191 Antiochus of Syria vainly attempted to hold the pass against the Romans under M'. Acilius Glabrio. In the time of Leonidas the pass was a narrow track (probably about 14 yd. wide) under the cliff. In modern times the deposits of the Spercheius have widened it to a breadth of II- to 3 m. broad. The hot springs from which the pass derived its name still exist close to the foot of the hill. There is one large spring used as a bath and four smaller ones, and the water, which is of a bluish green colour and contains lime, salt, carbonic acid and sulphur, is said to pro duce good effects in scrofula, sciatica and rheumatism.
In either form, expansion or contraction with change of tern perature affects fuel consumption and the production of heat.
In gas-fired hot water apparatus, in which a thermostat is com monly used, the regulators are worked by a capsule, containing volatile fluid, placed in close proximity to the water being heated; as the temperature rises the fluid expands and reduces the size of the passage through which the gas is conveyed to the burners. Figure i shows a section through such a thermostat. A small orifice or by-pass is generally provided, around the main gas pas sage, to maintain a small gas flame whatever condition prevails in the main passage. This prevents the gas from being entirely extin guished. See also REGULATION, AUTOMATIC.
The rod thermostat is another form of thermostat often used in gas apparatus. It is of very simple construction. As the tern perature rises the tube enclosing the rod lengthens, thereby draw ing the rod outward and reducing the gas-way.
When either form of thermostat is used, the process is reversed when water is drawn off. Upon cold water entering the apparatus the contraction of the thermostat allows more gas to pass, so that the temperature again rises. (See REGULATION, AUTOMATIC.)