he mind prevents slight sea-sickness. After a
voyage ideas of vibratory motions are still perceived on shore._ 6.
_Ideas continue some time after they are excited. Circumstances of
turning on one foot, standing on a tower, and walking in the dark,
explained._ 7. _Irritative ideas of apparent motions. Irritative ideas
of sounds. Battement of the sound of bells and organ-pipes. Vertiginous
noise in the head. Irritative motions of the stomach, intestines, and
glands._ 8. _Symptoms that accompany vertigo. Why vomiting comes on in
strokes of the palsy. By the motion of a ship. By injuries on the head.
Why motion makes sick people vomit._ 9. _Why drunken people are
vertiginous. Why a stone in the ureter, or bile-duct, produces
vomiting._ 10. _Why after a voyage ideas of vibratory motions are
perceived on shore._ 11. _Kinds of vertigo and their cure._ 12.
_Definition of vertigo._

1. In learning to walk we judge of the distances of the objects, which we
approach, by the eye; and by observing their perpendicularity determine our
own. This circumstance not having been attended to by the writers on
vision, the disease called vertigo or dizziness has been little understood.

When any person loses the power of muscular action, whether he is erect or
in a sitting posture, he sinks down upon the ground; as is seen in fainting
fits, and other instances of great debility. Hence it follows, that some
exertion of muscular power is necessary to preserve our perpendicular
attitude. This is performed by proportionally exerting the antagonist
muscles of the trunk, neck, and limbs; and if at any time in our
locomotions we find ourselves inclining to one side, we either restore our
equilibrium by the efforts of the muscles on the other side, or by moving
one of our feet extend the base, which we rest upon, to the new center of
gravity.

But the most easy and habitual manner of determining our want of
perpendicularity, is by attending to the apparent motion of the objects
within the sphere of distinct vision; for this apparent motion of objects,
when we incline from our perpendicularity, or begin to fall, is as much
greater than the real motion of the eye, as the diameter of the sphere of
distinct vision is to our perpendicular height.

Hence no one, who is hood-winked, can walk in a straight line for a hundred
steps together; for he inclines so greatly, before he is warned of his want
of p