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The rotatory motion of the earth about its axis produces a protuberant form in the equatorial regions of the earth; and the continued action of the Sun and Moon on this surrounding mass, or annulus, produces a rotatory motion in the axis of the earth round the imaginary axis of the ecliptic, the inclination of these supposed axés remaining perpetually the same. On this subject, which is rather difficult of apprehension, we shall not enlarge in the present volume ; only observing that the revolution here referred to will not be accomplished in less than 27,000 years.
The diürnal motion of the earth from west to east causes an apparent motion of the heavens from east to west. If the imaginary axis of the earth be prolonged each way 'till it intercepts the heavens, it will there form two points called the poles, round which the heavens appear to revolve. With us in the northern hemisphere, the north is the elevated pole, and the'stars near it will describe small circles, increasing with their distance from the pole, till they arrive at a certain distance, when a portion of the circle will be intercepted by the horizon. It will be evident, by te. ferring to an artificial celestial globe, that, as the polar distance of the stars increases, they will continue a longer time above the horizon ; still, however, rising and setting north of the east and west points of the horizon, till they reach the equator. At this distànce from the pole, they rise exactly in the east, and set exactly in the west ; and at their greatest altitude, their distance from the zenith is observed to be equal to the elevation of the pole above the horizon. Als the stars recede from the equator towards the south, they describe still smaller arcs, continue visible only for a few minutes, and no sooner 'appear on the meridian than they are carried down by the general motion of the heavens below the boundary of our sight. The Sun, Moon, and planets, appear to partake in a great measure of the diurnal motion of the heavens; they describe nearly the same course as a fixed star would do in the same situation.
The change from night to day is caused by the diurnal motion of the earth on its axis ; and the vicissitudes of the seasons, by the combinations of this motion with its annual revolution round the Sun. The annual motion of the earth round the Sun is performed in an orbit, the plane of which is inclined to the equator at an angle of 23° 28', and the effect of this motion is to cause the apparent annual motion of the Sun. When the earth is in any point of its orbit, the apparent place of the Sun will be in the direction of the opposite point, and the apparent path of the Sun will be similar to that actually traced by the earth; and these two bodies will always be in oppo. site positions to each other. It was not till astronomy had arrived at a state of great perfection, that sufficient proofs could be collected to ascertain, with any degree of precision, which of these bodies were really in motion. The fact has, however, been long since demonstrated with as much certainty as any proposition in Euclid. • The difference of seasons and of climates on the earth, and the inequality in the length of the days and nights, all arise from this circumstance, that the plane of the earth's orbit is inclined to that of the equator. If a straight line be supposed always to connect the centres of the earth and Sun, this straight line or ra. dius:vector will always trace the ecliptic in the heavens; and the point where it intercepts the surface of the earth will be that point over which the Sun is vertical. Now it will be evident, with a little consideration, that when the earth is in the upper or north part of its orbit, this line, supposed to pass through the centre of the Sun, will intercept some point to the south of the equator; and, on the contrary, when the earth is in the lower or southern part of its orbit, this line will intercept some point to the north of the equator : the circle bounding light and darkness being
always perpendicular to this line, the circles parallel to the equator will be unequally divided at different parts of the orbit.
The inequality in the distribution of the seasons, by which the inhabitants of the northern hemisphere of the globe enjoy a greater portion of the Sun by about eight days in the year, arises from a cause that shall now be illustrated.
The imaginary great circle in the heavens, called the ecliptic, is in truth the representation of the ellipse in which the earth travels round the Sun: all the irregularities, therefore, of the ellipse will, by observers, be transferred to the circle which represents it, and the four portions intercepted by the equinoctial and solstitial points, though represented each by an arc of 90°, will not be described in equal times, because they do not represent such portions of the ellipse as are originally so described. The point of the ecliptic, representing that point of the ellipse in which the Sun and earth are nearest to each other, is called the perigee; and the summer and winter months can never be equal, but in the singular instance of the coincidence of the perigee with one or other of the equinoctial points. The solar perigee has a progressive motion in the ecliptic, which it is found to complete in about 20,000 years; and it is a very remarkable circumstance, that this curious coincidence of the perigee with the autumnal equinox took place about the time when ehronologists suppose the world to have been created, or about the year 4000 before the birth of Christ. At that time, each hemisphere enjoyed an equal portion of the Sun's light and heat, and since then the northern hemisphere had gradually the ad. vantage, and the maximum of inequality took place in the year 1250. Since that time the advantage has gradually diminished, though it is very considerable. It will diminish till the year 6472, when the perigee, coinciding with the vernal equinox, will again equally divide the summer and winter periods between the
two hemispheres ; after which, the southern inhabitants of the globe will enjoy the advantage of having the greatest share of the Sun for above 10,000 years.
We shall resume the subject of the earth in our next month's discussions.
The Naturalist's Diary.
Hail, bounteous May, that dost inspire
This month is usually considered as the most delightful of the whole year, and has long been the Muse's favourite theme; although much that is said of its beauties applies better to more southern climates, or, indeed, to our month of JUNE, which is, commonly, entitled to all the praises that the poets have-lavished upon May. This month, however, is remarkable for the profusion of verdure which it exhibits : Nature's carpet is fresh laid, and nothing can be more grateful than to press its velvet surface:
When May is in his prime,
Then may each heart rejoice ;
Each bird strains forth his voice.
Into the blooming thorn;
Now laugh the frost to scorn,
Whiles joyful May doth last;
The pleasant time is past?. The scenery of a May morning is, not unfrequently, as beautiful as can possibly be conceived ; a serene
2. Paradise of Dainty Devices, Ed. 1576.
sky, a pure air, a refreshing fragrance arising from the face of the earth, and the inelody of the feathered tribes, all combine to render it inexpressibly delightful, to exhilarate the spirits, and to call forth a song of grateful adoration.
The latest species of the summer birds of passage arrive about the beginning of this month. (1) The goatsucker, or fern-owl (caprimulgus Europeus), makes its appearance only in the dusk of the evening, to search for prey, and, while perched on a tree, utters a dull jarring noise'. It feeds on insects, and is a great destroyer of the cock-chaffer or dor-beetle. This bird is found in every part of the old Continent, from Siberia to Greece, Africa, and India; it leaves England about the end of August. (2) The spotted Ay-catcher (musicapa grisola) is the most mute and familiar of all our summer birds; it builds in a vine or sweet briar, against the wall of a house, or on the end of a bearn, and sometimes close to the post of a door, where people are going in and out all day long : it returns to the same place year after year. The sedge-bird (motacilla salicaria) is found in places where reeds and sedges grow, and buildsits nest there, which is made of dried grass, tender fibres of plants, and lined with hair. It sings incessantly, night and day, during the breeding time, and imitates, by turns, the notes of the sparrow, the skylark, and other birds, from which it is called the English mock-bird.
The arduous time of incubation is now come, and birds are sedulously employed in hatching and rearing their young :
Each secret nook, each silent shade
1 Thence termed the night-jar by Mr. Bewick.
2 The pied fly-catcher (muscicapa atricapilla) is nowhere common, but is most plentiful in Yorkshire, Lancashire, and Derbyshire.