It was formerly required for each marine boiler to have a small valve
loaded with a weight to a few pounds per square inch above the working
pressure, so that in case of the safety valves sticking fast and the
gauge being false, an alarm might be given when there was an excess of
pressure. Such valves were about 3/4 inch in diameter and sometimes
as small as 3/8. An arrangement of a small safety valve attached to
a whistle has been introduced, but with advances in other directions
relating to safety these specialties are now getting to be only known
by name.