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reality no effect on the seasons; for the supply of heat to any place depends not only on the amount imparted to it in a given time, but on the length of time during which it continues to be received. Now the time during which the sun shines on any place is modified by the velocity of the earth in that part of its orbit in which it happens to be; because, as the earth's distance from the sun increases, its angular velocity diminishes; and thus the time during which the heat is communicated is increased, the injurious effect of the sun's augmented distance being exactly counterbalanced by the diminished velocity with which it travels through a given distance, and the resulting greater length of time during which its rays are received. This is a consequence of one of the laws discovered by Kepler. According to this law, “the spaces described by a line joining the centres of the sun and the earth, or any other planet, are equal in equal times,” no change being caused by any alteration in the planet's position in its orbit, the angular velocity being least when the distance from the sun is greatest, and vice versâ. The rays of the sun, intercepted by the earth, in equal times, no matter where it may be in its orbit, are therefore always equal in quantity.
But for this admirable compensation the difference be. tween summer and winter would be greatly augmented in the southern and lessened in the northern hemisphere, since the earth's distance from the sun fluctuates to the extent of nearly the one-fifteenth of its entire amount. The direct impression of the solar heat is, however, in the height of summer, under a perfectly clear sky at noon, greater in a southern than in a corresponding northern latitude; and hence the sufferings of travellers in the thirsty deserts of Australia are greater than in those of Africa.
The earth is retained in its orbit by two forces, one of which-centripetal-prevents it from flying off into infinite space; and the other-centrifugal, which was originally that of projection-prevents it from being precipitated upon the sun. And, just as the former of these, during the earth’s revolution in its orbit, is in.
creased by the earth's approximation to the sun, the latter, which is required to counteract it, is increased also. Each becomes predominant alternately, but neither retains its superiority long enough to produce an injurious effect. The very velocity with which the earth, in one part of its orbit, rushes towards the sun, generates a force which carries it away from that body; and the velocity with which it flies from the sun is gradually lessened by centripetal force, until at length the latter becomes sufficiently powerful to turn it back again towards that luminary. If the direction and amount of the force with which the earth was first projected, so as to cause it to revolve round the sun, had been such as to produce an elliptical orbit of great eccentricity—that is, a very long and narrow ellipse—the earth, during part of its revolution in such an orbit, would be so far from the sun as to derive but little benefit from it.
The rotation of the earth on its axis is necessary to produce alternations of day and night; and the revolution of the earth round the sun, in an orbit oblique to the equator, is required for the changes of the seasons. Were the variation of this orbit from the circular form not compensated by a change of velocity depending on the mutual action of the centripetal and centrifugal forces, or were the obliquity of the ecliptic either greater or less than it is, we should be subjected to extremes of temperature which, if not fatal to our existence, would greatly affect our comfort.
Thus, in these, as in all other circumstances connected with this earth and whatever it contains, not only our preservation, but our happiness has been carefully provided for.
EXERCISE.-71. COMPOSITION. 1. Describe the manner in which the phenomena of day and night are produced.
2. What is meant by the earth’s orbit? What is the cause of its being elliptical rather than circular ?
3. Account for the cold being greater in the southern hemisphere than 4. What is meant by an oblate spheroid? Why is the earth of this shape ? 5. Explain the terms centrifugal and centripetal, as applied to the
in the northern.
THE VETERAN TAR.
DAVID MACBETH MOIR. *
com-pelled (L. com, intensive; pello, to drive), forced, obliged. reclined (L. re, back; clino, to lean, from Gk. klino, to bend], leant back, resting in a recumbent posture. vol-un-teer (L. voluntas, will, from volo, to desire, to will], one who undertakes any duty of his own free will,
A MARINER, whom fate compelled
To make his home ashore,
With ivy mantled o'er ;
The sound of ocean's roar.
To mark how stood the wind :
Brought back old times to mind,
The sunny deck reclined.
All ocean plants were met-
With scented mignonette ;
Sea-thistles freaked with jet.
Rigged out in gallant style ;
And Nelson at the Nile,
When lonely, to beguile.
And there were charts and soundings made
By Anson, Cook, and Bligh ; Fractures of coral from the deep,
And stormstones from the sky; Shells from the shores of gay Brazil ;
Stuffed birds, and fishes dry.
No relative had he;
A haunter of the quay ;
He took him to the sea.
He sailed—a growing lad ;
In endless summer clad,
To palmy Trinidad.
When, ʼmid the sea-fight's jar,
To crown the British tar ;
On board a ship of war.
He ploughed the changeful deep;
The winged fishes leap,
To everlasting sleep.
Methinks upon my view
Striped shirt, and jacket blue;
and plaited queue.
Yon turfen bench the veteran loved,
Beneath the threshold tree,
The broad expanse of sea,-
Had been a rover free!
And lighted up his faded face,
When drifting in the gale,
Far off, a coming sail :
music to his ear, To list the sea-mews' wail!
Oft would he tell how, under Smith,
Upon the Egyptian strand,
They joined the men on land,
Behind their bags of sand ;-
With Nelson in his might,
To quell the Dane in fight,-
His veteran eye with light!
The brave old man would speak;
A glow suffused his cheek,
Had left him worn and weak.
Ten years, in vigorous old age,
Within that cot he dwelt; Tranquil as falls the snow on snow,
Life's lot to him was dealt ;, But came infirmity at length,
And slowly o'er him stealt.