d magnet, as in the later siphon recorder of Sir William Thomson.
Bain also proposed to make the coil record the message by printing it in
type; and he developed the idea in a subsequent patent.

Next year, on December 31st, 1844, he projected a mode of measuring
the speed of ships by vanes revolving in the water and indicating their
speed on deck by means of the current. In the same specification he
described a way of sounding the sea by an electric circuit of wires,
and of giving an alarm when the temperature of a ship's hold reached a
certain degree. The last device is the well-known fire-alarm in which
the mercury of a thermometer completes an electric circuit, when it
rises to a particular point of the tube, and thus actuates an electric
bell or other alarm.

On December 12, 1846, Bain, who was staying in Edinburgh at that time,
patented his greatest invention, the chemical telegraph, which bears his
name. He recognised that the Morse and other telegraph instruments in
use were comparatively slow in speed, owing to the mechanical inertia
of the parts; and he saw that if the signal currents were made to pass
through a band of travelling paper soaked in a solution which would
decompose under their action, and leave a legible mark, a very high
speed could be obtained. The chemical he employed to saturate the paper
was a solution of nitrate of ammonia and prussiate of potash, which left
a blue stain on being decomposed by the current from an iron contact or
stylus. The signals were the short and long, or 'dots' and 'dashes' of
the Morse code. The speed of marking was so great that hand signalling
could not keep up with it, and Bain devised a plan of automatic
signalling by means of a running band of paper on which the signals of
the message were represented by holes punched through it. Obviously
if this tape were passed between the contact of a signalling key the
current would merely flow when the perforations allowed the contacts of
the key to touch. This principle was afterwards applied by Wheatstone in
the construction of his automatic sender.

The chemical telegraph was tried between Paris and Lille before a
committee of the Institute and the Legislative Assembly. The speed of
signalling attained was 282 words in fifty-two seconds, a marvellous
advance on the Morse electro-magnetic instrument, which only gave about
forty words a minute. In the hands of Edison the neglected method of
Bain was seen by Sir William T