LETTER XI.

OF THE DIFFERENT LENGTHS OF DAYS AND NIGHTS, AND THE VICISSITUDES OF THE SEASONS.

NATURE is always grand in her designs, but frugal in the execution of them: sublimity and simplicity are the striking characteristics of her operations. From a few simple principles she produces the most astonishing effects, and charms us no less by the infinite diversity of her operations, than by the skill and contrivance which are manifested in the performance of them. The sun, moon, planets and stars, are all governed by the same invariable laws; the single principle of gravitation pervades the whole universe, and puts every spring and wheel of it in motion. From the indiscernible atom, to the vast and immeasurable luminaries of heaven, every thing is subject to its dominating influence; and from this active, invisible, and invigorating agent, proceeds all that order, harmony, beauty and variety, which so eminently distinguishes the works of creation.

But of all the effects resulting from this admirable scene of things, nothing can be more pleasing and agreeable to a philosophic mind, than the alternate succession of day and night, and the regular return of the seasons.

"Sweet is the breath of morn,

And sweet the coming on of grateful evening mild."

When the sun first appears in the horizon, all na

ture is animated by his presence; the magnificent theatre of the universe opens gradually to our view, and every object around us excites ideas of pleasure, admiration and wonder. After "riding in all his brightness" through the vault of heaven, he is again hid from our sight, and we are now presented with a new spectacle of equal grandeur and sublimity. The heavens are on a sudden covered with innumerable stars; "the moon rising in clouded majesty, unveils her peerless light;" whilst the silent solemnity of the scene, fills the mind with sentiments and ideas beyond the power of language to express,

Variety is the source of every pleasure; and the bountiful Author of nature, in the magnificent display of his wisdom and power, has afforded us every possible means of entertainment and instruction. What a pleasing succession of scenes results from the gradual vicissitudes of the seasons? Summer, winter, spring and autumn, lead us insensibly through the varied circle of the year; and are no less pleasing to the mind, than necessary towards bringing to maturity the various productions of the earth. Whether the sun flames in the solstice, or pours his mild effulgence from the equator, we equally rejoice in his presence, and bless that omniscient Being who gave him his appointed course, and prescribed the bounds which he can never pass.

These phænomena depend upon the most simple and evident principles; and as you will naturally be desirous of knowing in what way they are effect

ed, I shall omit all further digressions, and proceed immediately to the illustration of the subject. In the first place, then, it is to be observed, that the alternate succession of day and night is occasioned merely by the uniform rotation of the earth upon its axis. For, as the globe turns regularly round upon this imaginary line, once in every twenty-four hours, and only one half of it can be illuminated at a time, it is evident that any particular place will sometimes be turned towards the sun, and sometimes from it, and being constantly subject to these various positions, will enjoy a regular return of light and darkness; as long as the place continues in the enlightened hemisphere it will be day, and when, by the diurnal rotation of the earth, it is carried into the dark hemisphere, it will be night.

The motion of the earth upon its axis, is from west to east; and this occasions an apparent motion of the celestial bodies in a contrary direction. The sun, for instance, seems to make his daily progress through the heavens from the east towards the west; but this is an optical delusion, arising from the opposite motion of the earth for a spectator being placed in any part of the dark hemisphere, will, by the rotation of the earth upon its axis, be brought gradually into the enlightened one; and as the sun first appears to him in the east, it will seem to ascend higher and higher towards the west, in proportion as the spectator moves in a contrary direction towards the east: so that whether the earth turns round upon its axis. once in twenty-four hours, or whether the sun and

all the other celestial bodies, move round the earth in that time, the appearances will be exactly the same.

Every planet, whose situation is such as to admit of the necessary observations to be made on it, has been found to have a revolution upon its axis; and as this revolution is the cause of a constant succession of day and night to every part of their surfaces, so an inclination of the axis of any planet to the plane of its orbit, occasions the vicissitudes of the seasons. Thus, Jupiter, whose axis is nearly perpendicular to the plane of his orbit, has equal days and nights continually, from one pole to the other; their length being each four hours and twenty-eight minutes: but Venus, the Earth, and also Mars, according to the late discoveries of Dr. Herschel, having their axes inclined to the planes of 'their orbits, in an angle considerably less than that of ninety degrees, are subject to an annual change of their seasons, and to a great variety in the length of their days and nights.

As only one half of the globe can be enlightened at a time, the circle which is the boundary of light and darkness may be called the terminator; and it is evident, from a slight consideration of the subject, that if the axis of the earth was perpendicular to the plane of its orbit, the terminator would pass through the two poles, and divide each of the small circles, which are drawn parallel to the equator, into two equal parts. And as the uniform rotation of the earth upon its axis, must occasion every place to describe equal parts of one of these parallel

circles in equal times, the days and nights would, of course, be equal all over the globe, except at the poles, where the sun would neither rise nor set, but remain continually in the horizon.

But, on the contrary, if the axis of the earth be inclined to the plane of its orbit, all the parallels, except the equator, will be divided by the terminator into two unequal parts, having a greater or less portion of their circumferences in the enlightened, than in the dark hemisphere, according to their respective situations on the globe, and the place of the earth in its orbit. So that those places situated in either hemisphere, which have their pole turned towards the sun, will have their days longer than their nights; and on the contrary, those places which lie in the opposite hemisphere, will have their nights longer than their days; whilst at the equator, the days and nights will be continually equal to each other.

But since the axis of the earth always remains parallel to itself, when the earth is in the opposite point of its orbit, the contrary pole will be turned towards the sun, and the parallels will be still unequally divided by the terminator; but the phænomena will be directly the contrary; all those places situated within the latter hemisphere, will now have their days longer than their nights, whilst those in the former will have their nights longer than their days; the days and nights at the equator being equal to each other, the same as before. And, as the disproportion is greatest in the higher latitudes, it is evident, that in either of these two