the various phenomena of sleep are exhibited in greater regularity and perfection. Sleep seems more indispensably requisite to man than to any other creature, if there can be supposed to exist any difference where its indispensability is universal, and where every animal must, in some degree or other, partake of it; but, as regards man, it is certain that he sustains any violation of the law ordaining regular periods of repose with less indifference than the lower grades of creation—that a certain proportion of sleep is more essential to his existence than theirs-that he has less power of enduring protracted wakefulness, or continuing in protracted sleep-and that he is more refreshed by repose and more exhausted by the want of it than they. The sleep of man, therefore, becomes a subject of deeper interest and curiosity than that of any other animal, both on account of the more diversified manner in which it displays itself, and the superior opportunity which exists of ascertaining the various phenomena, which in the inferior animals can only be conjectured or darkly guessed at.

Sleep, being a natural process, takes place in general without any very apparent cause. It becomes, as it were, a habit, into which we insensibly fall at stated periods, as we fall into other natural or acquired habits. But it differs from the latter in this, that it cannot in any case be entirely dispensed with, although by custom we may bring ourselves to do with a much smaller portion than

we are usually in the practice of indulging in. In this respect it bears a strong analogy to the appetite for food or drink. It has a natural tendency to recur every twenty-four hours, and the periods of its accession coincide with the return of night.

But though sleep becomes a habit into which we would naturally drop without any obvious, or very easily discovered cause, still we can often trace the origin of our slumbers; and we are all acquainted with many circumstances which either produce or heighten them. I shall mention a few of these causes.

Heat has a strong tendency to produce sleep. We often witness this in the summer season; sometimes in the open air, but more frequently at home, and above all in a crowded meeting. In the latter case the soporific tendency is greatly increased by the impurity of the air. A vitiated atmosphere is strongly narcotic, and when combined with heat and monotony, is apt to induce slumber, not less remarkable for the rapidity of its accession than its overpowering character. In such a situation, the mind in a few minutes ceases to act, and sinks into a state of overpowering oblivion. The slumber, however, not being a natural one, and seldom occurring at the usual period, is generally short: it rarely exceeds an hour; and when the person awakes from it, so far from being refreshed, he is unusually dull, thirsty, and feverish, and finds more than common difficulty in geting his mental powers into their usual state of activity.

A heated church and a dull sermon are almost sure to provoke sleep. There are few men whose powers are equal to the task of opposing the joint operation of two such potent influences. They act on the spirit like narcotics, and the person seems as if involved in a cloud of aconite or belladonna. The heat of the church might be resisted, but the sermon is irresistible. Its monotony falls in leaden accents upon the ear, and soon subdues the most powerful attention. Variety, whether of sight or sound, prevents sleep, while monotony of all kinds is apt to induce it. The murmuring of a river, the sound of the Eolian harp, the echo of a distant cascade, the ticking of a clock, the hum of bees under a burning sun, and the pealing of a remote bell, all exercise the same influence. So conscious was Boerhaave of the power of monotony, that in order to procure sleep for a patient, he directed water to be placed in such a situation as to drop continually on a brass pan. When there is no excitement, sleep is sure to follow. We are all kept awake by some mental or bodily stimulus, and when that is removed our wakefulness is at an end. Want of stimulus, especially in a heated atmosphere, produces powerful effects; but where sufficient stimulus exists, we overcome the effects of the heat, and keep awake in spite of it. Thus, in a crowded church, where a dull, inanimate preacher would throw the congregation into a deep slumber, such a man as Massilon, or Chalmers, would keep them in a state of keen excitement.

He would arrest their attention, and counteract whatever tendency to sleep would otherwise have existed. In like manner, a prosing, monotonous, long-winded acquaintance is apt to make us doze, while another of a lively, energetic conversation keeps us brisk and awake. It will generally be found that the reasoning faculties are those which are soonest prostrated by slumber, and the imaginative the least so. A person would more readily fall asleep if listening to a profound piece of argumentation, than to a humorous or fanciful story; and probably more have slumbered over the pages of Bacon and Locke, than over those of Shakspeare and Milton.

Cold produces sleep as well as heat, but to do so a very low temperature is necessary, particularly with regard to the human race; for, when cold is not excessive, it prevents, instead of occasioning slumber in illustration of which, I may mention the case of several unfortunate women, who lived thirty-four days in a small room overwhelmed with the snow, and who scarcely slept during the whole of that period. In very northern and southern latitudes, persons often lose their lives by lying down in a state of drowsiness, occasioned by intense cold. The winter sleep, or hybernation of animals, arises from cold; but as this species of slumber is of a very peculiar description; I have discussed it separately in another part of the work. The finished gratification of all ardent desires has the effect of inducing slumber; hence, after

any keen excitement, the mind becomes exhausted, and speedily relapses into this state. Attention to a single sensation has the same effect. This has

been exemplified in the case of all kinds of monotony, where there is a want of variety to stimulate the ideas, and keep them on the alert. "If the mind," says Cullen, "is attached to a single sensation, it is brought very nearly to the state of the total absence of impression;" or, in other words, to the state most closely bordering upon sleep. Remove those stimuli which keep it employed, and sleep ensues at any time.

Any thing which mechanically determines the blood to the brain, acts in a similar manner, such as whirling round for a great length of time, ascending a lofty mountain, or swinging to and fro. The first and last of these actions give rise to much giddiness, followed by intense slumber, and at last by death, if they be continued very long. By lying flat upon a millstone while performing its evolutions, sleep is soon produced, and death, without pain, would be the result, if the experiment were greatly protracted. Apoplexy, which consists of a turgid state of the cerebral vessels, produces perhaps the most complete sleep that is known, in so far that, while it continues, it is utterly impossible to waken the individual: no stimulus, however powerful, has any influence in arousing his dormant faculties. When the circulating mass in the brain is diminished beyond a certain extent, it has the same effect on the oppo