it is yet most certainly so, and so in all the senses.  Every one knows that sleep is a relaxation; and that silence, where nothing keeps the organs of hearing in action, is in general fittest to bring on this relaxation; yet when a sort of murmuring sounds dispose a man to sleep, let these sounds cease suddenly, and the person immediately awakes; that is, the parts are braced up suddenly, and he awakes.  This I have often experienced myself, and I have heard the same from observing persons.  In like manner, if a person in broad daylight were falling asleep, to introduce a sudden darkness would prevent his sleep for that time, though silence and darkness in themselves, and not suddenly introduced, are very favorable to it.  This I knew only by conjecture on the analogy of the senses when I first digested these observations; but I have since experienced it.  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.  Black will always have something melancholy in it, because the sensory will always find the change to it from other colors too violent; or if it occupy the whole compass of the sight, it will then be darkness; and what was said of darkness will be applicable here.  I do not purpose to go into all that might be said to illustrate this theory of the effects of light and darkness; neither will I examine all the different effects produced by the various modifications and mixtures of these two causes.  If the foregoing observations have any foundation in nature, I conceive them very sufficient to account for all the phenomena that can arise from all the combinations of black with other colors.  To enter into every particular, or to answer every objection, would be an endless labor.  We have only followed the most leading roads; and we shall observe the same conduct in our inquiry into the cause of beauty.