1. Sleep is the periodical and temporary suspension of those functions that connect us with the external world. Man is so constituted, that the functions of sensibility, voluntary motion, and the intellectual faculties cannot be indulged for any length of time without fatigue. The nervous energy, which seems essential to their exercise, becomes exhausted ; the muscles can no longer contract; the external senses cannot receive impressions; the brain, and consequently the mind, becomes torpid; and a person sinks into a state of torpor and inaction, called sleep,

2. The approach of sleep is announced by an internal sensation, termed drowsiness, which gradually increases in strength, till at length it becomes irresistible. Great languor of the muscles and heaviness of the eyes are experienced; the sight yields first; next the smell; then the hearing ; and lastly, the sense of touch ; while at the same time, all the internal sensations, such as hunger and thirst, are no longer felt. The will ceases to control the functions that are under its influence, till finally all power of volition is wholly lost. Respiration is still carried on,, but chiefly by means of the diaphragm, which is an involuntary muscle; it is, however, somewhat slower, as well as the circulation, than in the waking condition.

3. During perfect sleep, the functions of the brain are entirely suspended. In such a state, dreams do not happen ; for as the brain is in a state of complete inaction, the intellectual operations are consequently dormant. When the brain is not in a state of complete repose, objects and images may float confusedly through the mind, which are often the result of external impressions imperfectly perceived, as they excite but an imperfect reaction in the brain.

4. The organic or nutritial functions continue during sleep, but with diminished energy. The circulation and respiration are not only slower, but digestion is retarded, and secretion, nutrition, and calorification, are less active than when awake. The temperature of the body is also lowered during sleep, which may perhaps be owing to the facts above mentioned.

5. The duration of sleep is influenced by a variety of circumstances. The average time of regular, periodical sleep, in adults, is from five to eight hours. Infants require twice as much sleep as grown persons. The quantity of sleep required, depends very much upon habit. It is said of Pichegru, one of Buonaparte's generals, that, for a whole year, he had not more than one hour of sleep in twenty four hours. Buonaparte himself, when on active duty, seldom slept more than three hours in twenty four; often but one hour. Men of active minds, who are engaged in a series of interesting employments, sleep much less than the lazy and the listless. The intellectual and moral faculties seem to require a longer period of repose than the functions of voluntary motion.

6. Though the night is the proper season for sleep, owing to the absence of light, diminished warmth, its comparative stillness, as well as the exhaustion caused by the labours of the day, yet some animals, which pursue their prey by night, sleep during the day, as the cat, fox, wolf, otter, owl, and bat. Hybernating animals sleep for several months during the winter, such as the bear, hedge hog, marmot, etc. Some birds, also, such as bats and swallows, sleep during the win. ter, or hybernate. During this state their temperature is diminished, their secretions nearly checked, their excretions suppressed, their respiration slow, and scarcely perceptible, their circulation very languid, and sensibility to external impressions entirely suspended. The arterial blood of hybernating animals, diners but little from venous blood.

7. Dreams are now considered by physiologists to be owing to an irregular action of the brain, in which the controlling power of the will is lost, and the memory and the imagination bear unlimited sway. Indeed all the faculties of the mind may be in exercise. The mind reasons, judges, wills, and experiences all the various emotions. We seem to hear, see, walk, talk, and perform the usual offices of life. Sometimes the voluntary muscles are thrown into action, and the dreamer moves, speaks, groans, cries, or sings; but the moment consciousness is roused, he is awake. Dreams, therefore, have well been said to be " a portion of animal life, escaping from the torpor in which the rest of it lies buried."

8. Before the functions of the brain were understood, dreams were regarded as supernatural, and even now are considered in this light by the ignorant and superstitious. Mr. Baxter, as well as Bishop Newton divided them into, two kinds, good and evil, because they believed that good and evil spirits were concerned in their production, according as one or the other obtained the ascendancy. It is supposed that animals dream, especially the dog; are they subject to supernatural influence ?

9. It is a singular circumstance in relation to our dreams, that we mistake our ideas for actual perceptions, and suppose that the train of images that passes through our minds, represents scenes which really exist. This is doubtless owing to the fact that, during sleep, the senses do not admit external impressions, and of course we are unable to compare the ideas that arise in our minds with sensible impressions, and thus perceive the difference between them. Our only consciousness consists in the images and ideas actually present in the mind; and it is therefore unavoidable that we should believe that our ideas represent objects actually existing.

10. There is a kind of dreaming, in some cases of imperfect sleep, where the will retains its power over the muscles of voluntary motion, while the external senses remain buried in partial or complete repose. This is called somnambulism or sleep walking.

11. Many remarkable cases of somnambulism have been lately published, as the effects of animal magnetism; but in many of these, it is to be feared, credulity existed on one side, and imposition was practised on the other. That somnambulism, or a state nearly similar, is brought about by the practices of magnetizers in persons of acute sensibility, and highly excitable nervous temperament, would seem scarcely to admit of a doubt. Still, all such cases should be regarded with great circumspection, and every means employed to detect imposition and fraud.

12. Somnambulism seems to differ from the waking state, only in consciousness being absent. The person appears to enjoy the full exercise of all his faculties ; he can converse, walk, sing, compose verses, and perform various operations ; and yet in the waking state he has no recollection of what has occurred. In this respect somnambulism differs from dreaming, and resembles a morbid state; indeed, it is a state of disease analagous to trance, catalepsy, or epilepsy.

13. A state, the reverse of somnambulism, is called incubus, or nightmare. In this affection, a person feels a sense of weight and suffocation, as if there was a heavy load on his stomach or chest, and imagines that some frightful object is seated there. This is owing to some oppression of the digestive, circulatory, and respiratory organs, most usually occasioned by a late, hearty supper of indigestible food. Frightful dreams denote ill health, and are often caused by a derangement of some important organ, as of the heart, stomach, or liver, exciting the brain to sympathetic action. To dream immediately on going to sleep, is always a mark of disease.

14. There is still another state which bears a close resemblance to somnambulism, and this is termed revery, absence of mind, or brown study. In this, the attention is so completely riveted to some particular subject, that the person is entirely insensible to the presence of surrounding objects.

15. One great object of education is to give the will power over the attention. The mind that cannot command this faculty, is in a deplorable condition for all the higher purposes of reflection and knowledge for which it is intended. Without it, perception exercises itself in vain ; the memory can lay up no store of ideas; the judgment draw forth no comparisons ; the imagination must become blighted and barren; and in proportion as a person has no command over his attention, in that degree must he be an idiot.

16. It is the will of Providence that all organized beings must perish. For a while, life maintains a successful conflict with the chemical laws of matter ; but at length its resistance becomes weaker and weaker, till, sooner or later, those functions cease which have enabled us to withstand the destructive influences by which we are surrounded.

17. The duration of life varies, not only in the animal but in the vegetable kingdom. As a general rule, where the growth is slow, the period of decrease is proportionably slow; and where it is rapid, decay as rapidly supervenes. The gourd that sprang up in a night, perished also in a night.

18. There are two kinds of death, natural and accidental. Natural death, or that of old age, is caused by a gradual decay and wearing out of all the organs ; accidental death, or such as is occasioned by accident, cuts off existence prematurely. The natural period of life differs in different individuals ; being influenced by a variety of circumstances; such as the original constitution; habits of life ; and the health of the locality where a person is situated.

19. The natural period of life, has not materially differed since the time of the flood. " The days of our years," says the Psalmist, " are threescore and ten ; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow, for it is soon cut off and we fly away." This description is as applicable now, as it was four thousand years ago.

20. Those who die from old age are few; being, in the city of New York, but one in thirty four. In New England, generally, the proportion is rather greater. The mean duration of life in New York is but twenty five years ; while the average rate of mortality, according to population, is one in thirty five. The diseases to which man is subject, from the earliest to the latest period of his existence, are numerous, and many of them of a fatal character. Sydenham estimates that two thirds of mankind die of acute diseases ; and that of the remaining, one third, or two ninths of the whole, die of consumption, leaving only one ninth to perish from other chronic maladies, and from pure old age.

20. As age approaches, the functions are all performed with less energy. The teeth fall out, and their sockets are absorbed ; respiration is not as readily accomplished; the valves of the heart, and the coats of many of the arteries become changed into bone, thus obstructing the circulation, and often causing an intermittent pulse; nutrition languishes ; the senses are blunted ; animal heat diminishes, so that warmer clothing is demanded ; the muscular system loses its power, and the body bends forward ; the limbs totter, the mental as well as corporeal faculties often fail; and the individual is reduced to second childhood, so well described by Shakspeare.

" Last scene of all, That ends this strange, eventful history, Is second childishness, and mere oblivion ; Sans teeth, Sans eyes, Sans taste, Sans everything."

As the other functions cease their office, sensibility gradually becomes extinct, and life almost imperceptibly takes its flight, vVO-' ' U'i*,