THIRD WEEK-THURSDAY.

ALTERNATION OF DAY AND NIGHT.

WITH us the sun now remains little more than eight hours above the horizon; and, even during that short period, the comparative intensity of his light and heat is much decreased. He is daily taking a less extensive circuit in the heavens; and in another month the length of the day will be diminished by more than another hour. Were the influence of the sun and the length of the day to continue in this state, the whole organized world, in the climate which we inhabit, would quickly be destroyed. But the year will soon recommence its annual round; and nature is even now preparing for its coming labours.

The repose of plants, and even of many animals, in this dreary season, reminds us of the salutary provision, of a similar description, which is made for the diurnal recruiting of exhausted strength by the alternation of night with day. That this arrangement is adapted to the constitution of animal and vegetable existences, will be readily admitted; and, on examining particulars, we shall be confirmed in our general conclusion. It is not merely true that nature requires a frequently recurring period of rest, but that the actual period of twenty-four hours, divided between activity and repose, is the best suited for this end. If this be so, it implies a Designing Cause; for such a period is arbitrary; that is to say, no reason can be assigned, in the nature of things, either why the earth should complete her daily revolution in twenty-four hours, or why animals and vegetables should require a season of rest in that precise interval. As to the former, no mechanical or physical necessity requires that our earth should complete three hundred and sixty-five revolutions in a year. It might, apparently, move either faster or slower, without the slightest inconvenience to the system with which it is connected. Jupiter and Saturn

revolve on their axis each in ten hours, which, considering their bulk, must carry their equators round with a velocity immensely greater than that of the earth, while Mercury which is so much nearer the sun, and so much smaller, has its day and night nearly of the same length as our own.

Now, if we look at the vegetable world, we shall find, as already observed, a remarkable adaptation of this arbitrary period of twenty-four hours to the constitution of its various productions. In some plants, indeed, this is not so obvious; but there are others which clearly show that they are endowed with a periodical character, corresponding with the average length of our day. Linnæus classified a number of plants according to their time of opening and shutting, with reference to the hour of the day, and found that there are some which change their hour of opening and shutting as the day becomes longer and shorter, while there are others which do not seem to be affected by the actual state of the light and heat, but have a daily period of their own, independent of these influences, expanding their leaves, and closing them, at a particular hour, whatever be the state of the weather, or the length of the day. Both of these instances prove an adaptation to the diurnal revolution of the earth, which could not be the effect of chance, and must, therefore, have been the work of an Intelligent Cause.

This adjustment is still more remarkable in the animal world. A period of sleep is necessary for the health and vigour of living beings; and the alternation of day and night, which actually takes place, seems, from various considerations, to be that which is best fitted for them; or, at all events, any very great deviation from the arrangement actually established, would be prejudicial. When a workman retires from his twelve hours' labour, he is sufficiently inclined to take rest; and, although it is possible for him to encroach on the night, without much inconvenience, and to extend his exertions, if not immoderate, to fifteen or sixteen hours, a longer period, without an interval of repose, would incur the risk of undermining the constitution. It is cer

tain, at least, that a considerable proportion of time, spent in sleep during the four and twenty hours, is of importance to the health both of body and mind; and that, if our day were extended, for example, to the length of two, the human powers would droop under the prolonged period which would thus occur between the intervals of rest.

To the lower animals, also, the alternation of day and night, as it actually exists, is wisely adapted. To some of these, the day is the season of collecting their food; to others, the night; but, whatever be the instincts which guide them in this respect, we cannot but perceive that the adjustment between their constitution and habits, on the one hand, and the period of light and darkness on the other, is such as to show that the one bears reference to the other, and to indicate benevolent contrivance.

It is no valid objection to this view, that the relative length of the days and nights varies very considerably in our climate, and still more in higher latitudes; because, where this s the case, we find adaptations and adjustments of a different kind, which, in some degree, compensate for these variations; and because, except in regions approaching very near the poles, the revolution of day and night is uniformly comprised in twenty-four hours; and the only difference consists in a longer or shorter time during which the curtains of night are drawn,—an inconvenience which the ingenuity of man removes by artificial means, and to which the habits and wants of the lower animals, and of plants, are wonderfully accommodated.

On the whole, we have here another proof of an Intelligent Creator, who has suited the organized beings he has called into existence, to the circumstances of the material world, in which he has been pleased to place them. The observations of Mr. Whewell on this subject, to whose judicious statements we have so frequently had occasion to refer, are entirely to the purpose. The hours of food and repose,' says he,' are capable of such wide modifications, in animals, and, above all, in man, by the influence of external stimulants and inter

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nal emotions, that it is not easy to distinguish what portion of the tendency to such alternations depends on original constitution. Yet no one can doubt that the inclination of food and sleep is periodical, or can maintain, with any plausibility, that the period may be lengthened or shortened without limit. We may be tolerably certain that a constantly recurring period of forty-eight hours, would be too long for one day of employment, and one period of sleep, with our present faculties; and all, whose bodies and minds are tolerably active, will probably agree, that, independently of habit, a perpetual alternation of eight hours up, and four in bed, would employ the human powers less advantageously and agreeably, than an alternation of sixteen and eight. A creature which could employ the full energies of his body and mind uninterruptedly for nine months, and then take a single sleep of three months, would not be a man.'

'This view,' he afterwards adds, 'agrees with the opinion of some of the most eminent physiologists. Thus Cabanis notices the periodical and isochronous character of the desire to sleep, as well as of other appetites. He states also, that sleep is more easy and more salutary, in proportion as we go to rest, and rise every day at the same hour, and observes, that this periodicity seems to have a reference to the motions of the solar system.'

All this leads to the conclusion, that the correspondence thus obvious between the laws of the material world, and the constitution of, man, and other animals, is not fortuitous, but is the appointment of a Wise Contriver, and manifests a designing First Cause.

THIRD WEEK-FRIDAY.

SLEEP.

THE remarkable manner in which the constitution of plants and animals is adapted to the length of the day, was yester

day commented on; and one of the most striking of these adaptations is the provision by which man, and many of the lower animals, drop into a state of inactivity and sweet oblivion during the night.

The presence of light is necessary to enable creatures, constituted as we are, to prosecute useful labours; but constant toil wears out the frame, and a period of rest is necessary. There is, therefore, an arrangement of our ever-provident Creator, by which light shall be, for a time, withdrawn from us, that we may be compelled, by a natural necessity, to refrain, at regulated and short intervals, from the prosecution of labours in which we might otherwise be too eagerly engaged. This is the point of view in which the subject has been already considered. Let us now attend to the subject in another light. Looking at the fact, that the earth is made to revolve on its axis once every twenty-four hours, and that thus its inhabitants are deprived of its light, and other genial influences, for nearly the half of that time, on an average, each day, What is the contrivance by which this natural occurrence is rendered agreeable and salutary? The reply is, that a provision is made by which the active powers both of body and mind are suspended, and sleep is induced.

And what is sleep? There is something very mysterious in this state, considered as a physiological phenomenon; but this inquiry does not fall under our present plan; and, if it did, we should probably find it difficult to come to any satisfactory conclusion as to its efficient cause, or the nature of the physical change in the nervous system, by which it is produced. We know it is a fact in the constitution of living beings; and this is all that it seems necessary, at present, to say on the subject. To define sleep according to its actual appearances, is sufficiently easy. In attending to our own experience, in regard to its approach and actual occurrence, we discover that the will seems gradually to become enfeebled in its power over both the bodily and mental operations; that the body becomes as it were benumbed, and ceases to receive impressions of external objects; and that the