"Fire a shot," said Ernest; "it may perhaps be heard, now that the air
is less humid."

The two-pounder was ready charged; Fritz struck a light and set fire
to a strip of mimosa bark, with which he touched the piece, and the
report boomed across the waters.

Willis raised his head and listened anxiously, but soon dropped it
again, and resumed his former attitude of hopeless despair.

"It may be," said Ernest, "that the _Nelson_ hears our signal, though
we do not hear hers."

"How can that be?" inquired Jack.

"Why, very easily. Sound increases or diminishes in intensity
according as the wind carries it on or retards it."

"What, then, is sound, that the wind can blow it about, most learned
brother?"

"It is a result of the compression of the air, that from its
elasticity extends and expands, and which causes a sort of trembling
or undulation, similar to that which is observed in water when a stone
is thrown into it."

"And you may add," said Becker, "that bodies striking the air excite
sonorous vibrations in this fluid; thus it rings under the lash that
strikes it with violence, and whistles under the rapid impulsion of a