th ease and rapidity,
while its discharges may be so grouped and combined as to give a most
important extension to the voice of the admiral in command. It is
needless to add that at any point upon our coasts, or upon any other
coast, where its establishment might be desirable, a fog-signal
station might be extemporised without difficulty.

*****

I have referred more than once to the train of echoes which
accompanied the explosion of gun-cotton in free air, speaking of them
as similar in all respects to those which were described for the first
time in my Report on Fog-signals, addressed to the Corporation of
Trinity House in 1874. [Footnote: See also 'Philosophical
Transactions' for 1874, p. 183.] To these echoes I attached a
fundamental significance. There was no visible reflecting surface
from which they could come. On some days, with hardly a cloud in the
air and hardly a ripple on the sea, they reached a magical intensity.
As far as the sense of hearing could judge, they came from the body of
the air in front of the great trumpet which produced them. The
trumpet blasts were five seconds in duration, but long before the
blast had ceased the echoes struck in, adding their strength to the
primitive note of the trumpet. After the blast had ended the echoes
continued, retreating further and further from the point of
observation, and finally dying away at great distances. The echoes
were perfectly continuous as long as the sea was clear of ships,
'tapering' by imperceptible gradations into absolute silence. But
when a ship happened to throw itself athwart the course of the sound,
the echo from the broadside of the vessel was returned as a shock
which rudely interrupted the continuity of the dying atmospheric
music.

These echoes have been ascribed to reflection from the crests of the
sea-waves. But this hypothesis is negatived by the fact, that the
echoes were produced in great intensity and duration when no waves
existed--when the sea, in fact, was of glassy smoothness. It has been
also shown that the direction of the echoes depended not on that of
waves, real or assumed, but on the direction of the axis of the
trumpet. Causing that axis to traverse an arc of 210 deg., and the
trumpet to sound at various points of the arc, the echoes were always,
at all events in calm weather, returned from that portion of the
atmosphere towards which the trumpet was directed. They could not,
under the circumstances, com