Messages can be conveyed by--
=1. Speaking tube;= for distances up to say 300 feet, ³⁄₄″ to 1″ bore
tubes.
=2. Telephone;= any distance.
=3. Telegraph;= any distance.
=4. By signals=--(a) Wire or cord and bell; (b) sight signals, such as
the semaphore, lamps, heliograph, flags, and other devices; (c) by
sound, such as a bell, trumpet, siren, whistle, &c. See Section 105.
=5. By pneumatic despatch:= that is, by forcing a piston carriage
containing the message or small parcel through a tube by compressed
air.
=6. Carrier pigeons.=
=368. Signalling dials= and =bevel gearing.=
[Illustration: 354.-365.]