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 perpendicularity 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