depended
on the nature of the conductor, its resistance, and its electro-static
capacity. Faraday showed, for example, that its velocity in a submarine
wire, coated with insulator and surrounded with water, is only 144,000
miles a second, or still less. Wheatstone's device of the revolving
mirror was afterwards employed by Foucault and Fizeau to measure the
velocity of light.

In 1835, at the Dublin meeting of the British Association, Wheatstone
showed that when metals were volatilised in the electric spark, their
light, examined through a prism, revealed certain rays which were
characteristic of them. Thus the kind of metals which formed the
sparking points could be determined by analysing the light of the spark.
This suggestion has been of great service in spectrum analysis, and as
applied by Bunsen, Kirchoff, and others, has led to the discovery
of several new elements, such as rubidium and thallium, as well as
increasing our knowledge of the heavenly bodies. Two years later,
he called attention to the value of thermo-electricity as a mode of
generating a current by means of heat, and since then a variety
of thermo-piles have been invented, some of which have proved of
considerable advantage.

Wheatstone abandoned his idea of transmitting intelligence by the
mechanical vibration of rods, and took up the electric telegraph. In
1835 he lectured on the system of Baron Schilling, and declared that the
means were already known by which an electric telegraph could be made of
great service to the world. He made experiments with a plan of his own,
and not only proposed to lay an experimental line across the Thames, but
to establish it on the London and Birmingham Railway. Before these plans
were carried out, however, he received a visit from Mr. Fothergill
Cooke at his house in Conduit Street on February 27, 1837, which had an
important influence on his future.

Mr. Cooke was an officer in the Madras army, who, being home on
furlough, was attending some lectures on anatomy at the University of
Heidelberg, where, on March 6, 1836, he witnessed a demonstration
with the telegraph of Professor Moncke, and was so impressed with its
importance, that he forsook his medical studies and devoted all his
efforts to the work of introducing the telegraph. He returned to London
soon after, and was able to exhibit a telegraph with three needles in
January, 1837. Feeling his want of scientific knowledge, he consulted
Faraday and Dr. Rog