soon instructs us with accuracy to discern
these modes of motion, and to ascribe the apparent motions of the ambient
objects to ourselves; but those, which we have not acquired by repeated
habit, continue to confound us. So as we ride on horseback the trees and
cottages, which occur to us, appear at rest; we can measure their distances
with our eye, and regulate our attitude by them; yet if we carelessly
attend to distant hills or woods through a thin hedge, which is near us, we
observe the jumping and progressive motions of them; as this is increased
by the paralax of these objects; which we have not habituated ourselves to
attend to. When first an European mounts an elephant sixteen feet high, and
whose mode of motion he is not accustomed to, the objects seem to undulate,
as he passes, and he frequently becomes sick and vertiginous, as I am well
informed. Any other unusual movement of our bodies has the same effect, as
riding backwards in a coach, swinging on a rope, turning round swiftly on
one leg, scating on the ice, and a thousand others. So after a patient has
been long confined to his bed, when he first attempts to walk, he finds
himself vertiginous, and is obliged by practice to learn again the
particular modes of the apparent motions of objects, as he walks by them.

4. A third difficulty, which occurs to us in learning to balance ourselves
by the eye, is, when both ourselves and the circumjacent objects are in
real motion. Here it is necessary, that we should be habituated to both
these modes of motion in order to preserve our perpendicularity. Thus on
horseback we accurately observe another person, whom we meet, trotting
towards us, without confounding his jumping and progressive motion with our
own, because we have been accustomed to them both; that is, to undergo the
one, and to see the other at the same time. But in riding over a broad and
fluctuating stream, though we are well experienced in the motions of our
horse, we are liable to become dizzy from our inexperience in that of the
water. And when first we go on ship-board, where the movements of
ourselves, and the movements of the large waves are both new to us, the
vertigo is almost unavoidable with the terrible sickness, which attends it.
And this I have been assured has happened to several from being removed
from a large ship into a small one; and again from a small one into a man
of war.

5. From the foregoing examples it is evident, that, when we