And I have often experienced, and so have a thousand others, that on the first inclining towards sleep, we have been suddenly awakened with a most violent start; and that this start was generally preceded by a sort of dream of our falling down a precipice: whence does this strange motion arise, but from the too sudden relaxation of the body, which by some mechanism in nature restores itself by as quick and vigorous an exertion of the contracting power of the muscles?
The dream itself is caused by this relaxation; and it is of too uniform a nature to be attributed to any other cause.

The parts relax too suddenly, which is in the nature of falling; and this accident of the body induces this image in the mind.

When we are in a confirmed state of health and vigor, as all changes are then less sudden, and less on the extreme, we can seldom complain of this disagreeable sensation.
SECTION XVIII.
THE EFFECTS OF BLACKNESS MODERATED.
Though the effects of black be painful originally, we must not think they always continue so.

Custom reconciles us to everything.

After we have been used to the sight of black objects, the terror abates, and the smoothness and glossiness, or some agreeable accident of bodies so colored, softens in some measure the horror and sternness of their original nature; yet the nature of the original impression still continues.