erpendicularity by the sense of touch, not having the apparent motions
of ambient objects to measure this inclination by, that he is necessitated
to move one of his feet outwards, to the right or to the left, to support
the new centre of gravity, and thus errs from the line he endeavours to
proceed in.

For the same reason many people become dizzy, when they look from the
summit of a tower, which is raised much above all other objects, as these
objects are out of the sphere of distinct vision, and they are obliged to
balance their bodies by the less accurate feelings of their muscles.

There is another curious phenomenon belonging to this place, if the
circumjacent visible objects are so small, that we do not distinguish their
minute parts; or so similar, that we do not know them from each other; we
cannot determine our perpendicularity by them. Thus in a room hung with a
paper, which is coloured over with similar small black lozenges or
rhomboids, many people become dizzy; for when they begin to fall, the next
and the next lozenge succeeds upon the eye; which they mistake for the
first, and are not aware, that they have any apparent motion. But if you
fix a sheet of paper, or draw any other figure, in the midst of these
lozenges, the charm ceases, and no dizziness is perceptible.--The same
occurs, when we ride over a plain covered with snow without trees or other
eminent objects.

2. But after having compared visible objects at rest with the sense of
touch, and learnt to distinguish their shapes and shades, and to measure
our want of perpendicularity by their apparent motions, we come to consider
them in real motion. Here a new difficulty occurs, and we require some
experience to learn the peculiar mode of motion of any moving objects,
before we can make use of them for the purposes of determining our
perpendicularity. Thus some people become dizzy at the sight of a whirling
wheel, or by gazing on the fluctuations of a river, if no steady objects
are at the same time within the sphere of their distinct vision; and when a
child first can stand erect upon his legs, if you gain his attention to a
white handkerchief steadily extended like a sail, and afterwards make it
undulate, he instantly loses his perpendicularity, and tumbles on the
ground.

3. A second difficulty we have to encounter is to distinguish our own real
movements from the apparent motions of objects. Our daily practice of
walking and riding on horseback