derable distance from the vessel; and suppose the
current be rapidly made and broken by an interrupter; then the observer on
a second vessel provided with similar terminal conductors to the first,
but having a telephone instead of a dynamo, will be able to detect the
presence of the other vessel even at a considerable distance; and by
suitable modifications the direction of the other vessel may be found.
This conception Professor Bell has actually tried on the Potomac River
with two small boats, and found that at a mile and a quarter, the furthest
distance experimented upon, the sound due to the action of the interrupter
in one boat was distinctly audible in the other. The experiment did not
succeed quite so well in salt water. Professor Trowbridge then mentioned a
method which he had suggested some years ago for telegraphing across the
ocean without a cable, the method having been suggested more for its
interest than with any idea of its ever being put in practice. A conductor
is supposed to be laid from Labrador to Patagonia, ending in the ocean at
those points, and passing through New York, where a dynamo machine is
supposed to be included in the circuit. In Europe a line is to extend from
the north of Scotland to the south of Spain, making connections with the
ocean at those points, and in this circuit is to be included a telephone.
Then any change in the strength of the current in the American line would
produce a corresponding change in current in the European line; and thus
signals could be transmitted. Mr. Preece, of the English postal telegraph,
then gave an account of how such a system had actually been put into
practice in telegraphing between the Isle of Wight and Southampton during
a suspension in the action of the regular cable communication. The
instruments used were a telephone in one circuit, and in the other about
twenty-five Leclanche cells and an interrupter. The sound could then be
heard distinctly; and so communication was kept up until the cable was
again in working order. Of the two lines used in this case, one extended
from the sea at the end of the island near Hurst Castle, through the
length of the island, and entered the sea again at Rye; while the line on
the mainland ran from Hurst Castle, where it was connected with the sea,
through Southampton to Portsmouth, where it again entered the sea. The
distance between the two terminals at Hurst Castle was about one mile,
while that between the termi