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We see around us many examples of this kind of force. When a boy runs round a corner, there is a certain centrifugal force which impels him outwards, and he resists the impulse by inclining his body towards the corner. A horse galloping round the ring in a circus, leans, for a like reason, towards the centre of the ring. The rider does the same thing; if he tried to stand erect, he would soon be thrown off the horse, and probably out of the ring altogether.
We can now understand why the moon does not fall. Its motion round the earth resembles, in many respects, that of a stone in a sling. Its orbit is nearly circular. The earth's attraction is the cord which prevents it from flying off at a tangent to that orbit. Its centrifugal force, again, prevents it from falling to the earth in obedience to that attraction, and the two together have the effect of impelling it onwards in its curvilinear path. Nor is it the moon alone which moves in accordance with these principles. On the contrary, the earth and moon together perform a similar revolution round the sun, and are kept in their proper place by an exact adjustment and balancing of the same two forces; their own centrifugal force on the one hand, and, on the other, the attraction of the sun, which, like every other attraction, acts always as a centripetal force. The planets, too, observe the same laws in their unceasing movements, and so does every revolving body in the universe. It is a beautiful characteristic of the works of God, that the grandest and most complicated results, alike with the simplest and most insignificant, are often traceable to the multiform operation of one or two plain, intelligible, comprehensive principles.
QUESTIONS FOR EXAMINATION.
Who first prominently announced the laws of motion? State them. Explain the effect of two forces acting on a body in the same directions-in opposite directions-in different, but not opposite directions. Give examples in each case. What is meant by action and reaction? Give examples. What is a vertical line? Are vertical lines parallel? Describe experiments showing that all bodies should fall with equal rapidity. Why do they not in ordinary circumstances? What is an accelerating force? What is the relation, in the case of a falling body, between the time, the velocity, and the space? How far will a body fall in six seconds? Describe the motion of a body projected vertically
downwards-vertically upwards-in any direction not vertical. Give examples of projectiles. What is the centre of gravity? Where is the centre of gravity of a sphere? What is the condition that determines whether a body will stand or fall? Give examples from inanimate objects-from men's postures. What two forces besides gravity act upon a stone whirled in a sling? What is centrifugal force? In what direction does it act? Give examples of its overcoming gravity. Describe the motion of a stone in a sling, and compare it with the motions of the moon and other heavenly bodies.
THE LAKE OF GENEVA.
CLEAR, plácid Lèman! thy contrasted lake, With the wild world I dwélt in, is a thing Which wárns me, with its stillness, to forsake Earth's troubled waters for a púrer spring. This quiet sáil' is as a noiseless wíng To waft me from distràction; ónce! I loved Torn òcean's roar, but thy soft múrmuring! Sounds sweet as if a sister's voice reproved, That I with stern delights' should e'er have been so mòved.
It is the hush of nìght, and álll between
Hél is an evening rèveller, who makes
Ye stárs! which are the poetry of heaven!
I'f in your bright leaves' we would read the fate
In us such love and reverence from afár,
All heaven and earth are still-though not in sleep,
Of that which is of áll' Creátor and defènce.
The sky is changed and such a change! Oh night,
And this is in the night! Most glòrious night: Thou wert not sent for slúmber! let mél be A shàrer in thy fierce and fàr delight,A portion of the témpest' and of thee! How the lit lake shines, a phosphoric sea, And the big ráin' comes dancing to the earth! And now again 'tis black-and now, the glee Of the loud hills' shakes with its mountain-mírth, As if they did rejoicel o'er a young earthquake's birth.-BYRON.
THE SIEGE OF TORQUILSTONE; FRONT-DE-BŒUF'S CASTLE.
WITH patient courage, Rebecca took post at the lattice, sheltering herself, however, so as not to be visible from beneath.
"What dost thou see, Rebecca?" again demanded the wounded knight.
"Nothing but the cloud of arrows flying so thick as to dazzle mine eyes, and to hide the bowmen who shoot them."
"That cannot endure," said Ivanhoe; "if they press not right on to carry the castle by pure force of arms, the archery may avail but little against stone walls and bulwarks. Look for the Knight of the Fetterlock, fair Rebecca, and see how he bears himself, for as the leader is, so will his followers be."
"I see him not," said Rebecca.
"Foul craven!" exclaimed Ivanhoe, "does he blench from the helm when the wind blows highest?"
"He blenches not! he blenches not," said Rebecca, “I see him now, he leads a body of men close under the outer barrier of the barbican. They pull down the piles and palisades; they hew down the barriers with axes. His high black plume floats abroad over the throng, like a raven over the field of the slain. They have made a breach in the barriers they rush in-they are thrust back! Front-deBoeuf heads the defenders; I see his gigantic form above the press. They throng again to the breach, and the pass is disputed, hand to hand, and man to man. It is the meeting of two fierce tides-the conflict of two oceans moved by adverse winds."
She turned her head from the lattice, as if unable longer to endure a sight so terrible.
"Look forth again, Rebecca," said Ivanhoe, mistaking the cause of her retiring; "the archery must in some degree have ceased, since they are now fighting hand to hand. Look again, there is now less danger."
Rebecca looked again, and almost immediately exclaimed,
"Holy prophets of the law! Front-de-Boeuf and the Black Knight fight hand to hand on the breach amid the roar of their followers, who watch the progress of the strife. Heaven strike with the cause of the oppressed and of the captive!" She then uttered a loud shriek, and exclaimed, "He is down! he is down!"
"Who is down?" cried Ivanhoe; "for our dear Lady's sake, tell me which has fallen.”
"The Black Knight," answered Rebecca faintly, then instantly again shouted with joyful eagerness, "But nobut no! the name of the Lord of hosts be blessed! he is on foot again, and fights as if there were twenty men's strength in his single arm. His sword is broken-he snatches an axe from a yeoman-he presses Front-de-Bouf with blow on blow. The giant stoops and totters like an oak under the steel of the woodman-he falls-he falls!" "The assailants have won the barriers, have they not?" said Ivanhoe.
"They have-they have!" exclaimed Rebecca, "and they press the besieged hard upon the outer wall; some plant ladders, some swarm like bees, and endeavour to ascend upon the shoulders of each other-down go stones, beams, and trunks of trees upon their heads, and as fast as they bear the wounded to the rear, fresh men supply their places in the assault. Great God! hast thou given men thine own image, that it should be thus cruelly defaced by the hands of their brethren!"
"Think not of that," said Ivanhoe, "this is no time for such thoughts. Who yield? Who push their way?"
"The ladders are thrown down," replied Rebecca shuddering, "the soldiers lie grovelling under them like crushed reptiles. The besieged have the better."
"Do the false yeomen give way?" exclaimed the knight. "No!" exclaimed Rebecca, "they bear themselves right yeomanly-the Black Knight approaches the postern with his huge axe-the thundering blows which he deals, you may hear them above all the din and shouts of the battle. Stones and beams are hailed down on the bold champion