These are of two kinds, known as the bell-whistle and organ-tube
whistle; the latter is now fast superseding the former on account of
the simplicity of construction and superior tone. An improved form has
a division in the tube so as to emit two distinct notes, which may be
in harmony, or discord, and when sounded together may be heard a long
distance.
It is important that the whistle shall sound as soon as the steam
is turned on; to ensure this great care must be taken to keep the
whistle-pipe free from water.