The Book of Household Management by Mrs. Beeton

561. THE TEMPERATURE AT WHICH WATER BOILS, under usual circumstances, is

212° Fahr. Water does not become hotter after it has begun to boil, however long or with whatever violence the boiling is continued. This fact is of great importance in cookery, and attention to it will save much fuel. Water made to boil in a gentle way by the application of a moderate heat is just as hot as when it is made to boil on a strong fire with the greatest possible violence. When once water has been brought to the boiling point, the fire may be considerably reduced, as a very gentle heat will suffice to keep the water at its highest temperature.