are surrounded
with unusual motions, we lose our perpendicularity: but there are some
peculiar circumstances attending this effect of moving objects, which we
come now to mention, and shall hope from the recital of them to gain some
insight into the manner of their production.

When a child moves round quick upon one foot, the circumjacent objects
become quite indistinct, as their distance increases their apparent
motions; and this great velocity confounds both their forms, and their
colours, as is seen in whirling round a many coloured wheel; he then loses
his usual method of balancing himself by vision, and begins to stagger, and
attempts to recover himself by his muscular feelings. This staggering adds
to the instability of the visible objects by giving a vibratory motion
besides their rotatory one. The child then drops upon the ground, and the
neighbouring objects seem to continue for some seconds of time to circulate
around him, and the earth under him appears to librate like a balance. In
some seconds of time these sensations of a continuation of the motion of
objects vanish; but if he continues turning round somewhat longer, before
he falls, sickness and vomiting are very liable to succeed. But none of
these circumstances affect those who have habituated themselves to this
kind of motion, as the dervises in Turkey, amongst whom these swift
gyrations are a ceremony of religion.

In an open boat passing from Leith to Kinghorn in Scotland, a sudden change
of the wind shook the undistended sail, and stopt our boat; from this
unusual movement the passengers all vomited except myself. I observed, that
the undulation of the ship, and the instability of all visible objects,
inclined me strongly to be sick; and this continued or increased, when I
closed my eyes, but as often as I bent my attention with energy on the
management and mechanism of the ropes and sails, the sickness ceased; and
recurred again, as often as I relaxed this attention; and I am assured by a
gentleman of observation and veracity, that he has more than once observed,
when the vessel has been in immediate danger, that the sea-sickness of the
passengers has instantaneously ceased, and recurred again, when the danger
was over.

Those, who have been upon the water in a boat or ship so long, that they
have acquired the necessary habits of motion upon that unstable element, at
their return on land frequently think in their reveries, or between
sleeping an