muscle must be in this class of animal. I have seen them spring out of
the water within ten yards of the ship's side, generally in the
evening, after having swam all the former part of the day in the
ship's _wake_, or on either quarter. When several of these fish take
it into their heads to dance a "hornpipe," as the sailors have termed
their gambols, at the distance of half a mile they, especially at or
just after sun-down, may easily be mistaken for the sharp points of
rocks sticking up out of the water, and the splashing and foam they
make and produce have the appearance of the action of the waves upon
rocks. An officer of the navy informed me, that after sunset, when
near the equator, he was not a little alarmed and surprised (because
quite unexpected) at the cry of "rocks on the starboard bow:" looking
forward through the dubious light (if the expression may be admitted,)
he indistinctly saw objects which he and all on board took to be the
pinnacles of several rocks of a black and white colour: in a short
time, however he discovered this formidable danger to be nothing more
than a company of dancing grampuses with white bellies: as one
disappeared, another rose, so that there were at least five or six
constantly above the surface!

The uncertainty attending the visual organ during the continuance of
the _aurora_ and of the _twilight_, must have been noticed by all
those person's who have frequented the ocean. Most sailors have the
power of eye-sight strengthened from constant practice, and from
having an unobstructed view so generally before them; yet I have known
an officer, who was famous for his quickness of sight, declare that in
the evening and morning he found it difficult to retain sight for more
than a second or two at a time, of a strange sail; at night, even with
an inverting glass, his practised eye could retain the object more
steadily.