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The first law of motion is stated thus :- A body in motion will continue to move in a straight line, with a uniform velocity, unless acted on by some external force. This is a necessary consequence of the inertia of matter, being equivalent to the assertion that matter possesses no inherent power of changing its state of rest or motion. But it is only by observation that we can ascertain the powers with which matter has been endowed, and, therefore, the truth of this law must be decided by an appeal to experiment. No direct experiment, however, will suffice to establish this law, since we have no opportunity of observing a body which is either at rest or in motion, and not at the same time subject to external force. A stone lying on the ground and one falling through the air are both acted on by the force of gravity and atmospheric resistance. A ball rolled along the ground is acted on by the same forces, as well as the force of friction. Experiment, however, will suffice to show that the more closely we approach to the prescribed condition of this first law of motion, by lessening the influence of external force, the more nearly does motion become direct and uniform. A ball rolled along a smooth and even road continues to move for a much longer time, and more nearly in a straight line, than when thrown with the same velocity along a rough road; and still more nearly does it fulfil this law when thrown along a sheet of ice. These approximations to the fulfilment of the first law of motion are owing to the diminished force of friction. But the resistance of the atmosphere is the same, whatever be the nature of the ground along which the body inores; and if this is diminished at the same time, as well as the force of friction, a still nearer approach will be made to the truth of this first law. A nicely adjusted wheel, when set in motion, will continue to revolve for a mueh longer time beneath the exhausted receiver of an air pump, than when subject to the ordinary resistance of the atmosphere. Under the same circumstances, the motion of an oscillating body is considerably prolonged. By thus diminishing the forces of friction and atmospheric resistance, successive approximations to continuous rectilinear motion may be made ; and as the moving body is always observed to move for a longer time, and more nearly in a straight line, in proportion to the diminution of external force, it may be reasonably inferred that if such force could be destroyed altogether, the body would continue to move for ever in a straight line, with a uniform velocity. But the most convincing evidence of the truth of the first law of motion is obtained from the strict accordance between the consequences deducible from it and observed astronomical phenomena. The time of an eclipse of the moon, or the transit of a planet, may be foretold within the fraction of a second; and this law is made the basis of such predictions.
The second law of motion states, that any change in the motion of a body is proportional to and in the same direction as the disturbing force. If one force deflect a body a certain distance from its original path, another force twice as great will bend it twice as far. If the force which acts thus on the body has a horizontal direction, the change in its motion will be in that direction; if it has a vertical direction, the body will turn towards the vertical; and similarly, for any other direction the body may have. A ball rolled along the deck of a sailing vessel moves in the same direction, as if the vessel were at rest. If the ball is let fall from the mast of the vessel, it is not left in the rear of the vessel but falls at the foot of the mast, precisely at the same place where it would strike the deck if the vessel were at rest. This second law of motion includes the parallelogram of velocities, a proposition occupying a similar position in dynamics to that of the parallelogram of forces in statics.
If pressure communicates motion to a body, the moving force generated is proportional to the pressure. This is the third law of motion. By the moving force of a body is meant the force with which it would strike against any object in its path, at any given time after it begins to move. Thus, if a body put in motion by a force of two pounds acting on it, strikes against a wall at the end of three minutes with a certain force, it will strike the wall with twice the force, at the end of the same time, if its motion is caused by a pressure of four pounds.
These laws lie at the foundation of inductive science, and their importance, in relation to all generalizations concerning motion, cannot be overrated.
LESSON XXIII.- WEDNESDAY.
THE SEA. Turn to the watery world !--but who to thee (A wonder yet unview'd) shall paint the Sea ? Various and vast, sublime in all its forms, When lull'd by zephyrs, or when roused by storms; Its colours changing, when from clouds and sun Shades after shades upon the surface run; Embrown’d and horrid now, and now serene, In limpid blue and evanescent green; And oft the foggy banks and ocean lie, Lift the fair sail, and cheat th' experienced eye.
Be it the summer noon! A sandy space The ebbing tide has left upon its place; Then just the hot and sandy beach above, Light twinkling streams in bright confusion move ; (For heated thus, the warmer air ascends, And with the cooler in its fall contends,)Then the broad bosom of the ocean keeps An equal motion, swelling as it sleeps ; Then slowly sinking; curling to the strand, Faint lazy waves o'ercreep the ridgy sand, Or tap the tarry boat with gentle blow, And back return in silence, smooth and slow. Ships in the calm seem anchor’d; for they glide On the still sea, urged solely by the tide: Art thou not present, this calm scene before, Where all beside is pebbly length of shore, And far as eye can reach, it can discern no more ?
Yet sometimes comes a ruffling cloud to make The quiet surface of the ocean shake; As an awaken'd giant with a frown Might show his wrath, and then to sleep sink down.
View now the winter storm! above, one cloud, Black and unbroken, all the skies o’ershroud; Th' unwieldy porpoise through the day before Had roll’d in view of boding men on shore; And sometimes hid, and sometimes show'd his form, Dark as the cloud, and furious as the storm.
All where the eye delights, yet dreads to roam,
Far off, the petrel in the troubled way
High o'er the restless deep, above the reach
In shore, their passage tribes of sea-gulls urge,
ENGLISH HISTORY—PLANTAGENET LINE. In 1459, a general rising was planned by the Yorkists, whose badge was the white rose, that of Lancaster being the red. At the battle of Northampton, 1460, the king was made prisoner, conveyed to London, and a compromise was effected, by which he was to retain the crown for life. and York was declared heir apparent. The queen refused to surrender the rights of her son; and, having collected an army, advanced to Wakefield, where the duke of York was slain, and his troops routed. The son of York maintained the strife with greater success. He gained a victory over the Lancastrians at Mortimer's cross; suffered, in his turn, a defeat at the second battle of St. Alban's; but continuing to press forward he entered London in triumph in 1461, and was proclaimed king, by the title of Edward IV.
Edward had accepted the offered crown; but he had yet to secure possession of it by his sword, and he lost no time in submitting his claims to its arbitration. The queen was at York, with a force of 60,000 men, and thither the new monarch directed his march. The armies met at Towton, and after a fatal conflict, extending over three days, the Lancastrians fled, half their number having perished. The queen, with her husband and son, fled to Scotland, and Edward entered York in triumph. Returning to London he was crowned with great pomp, assembled parliament, and applied himself to the administration of government, and the regulation of commerce. But the civil war was not yet ended. Margaret had been for some time in France, pleading her cause with Louis XI., and at length extorted from him some supplies of men and money. With these she landed in Northumberland, was defeated at the battle of Hexham, and once more found herself a fugitive. The unhappy Henry wandered for some time amidst the wilds of Cumberland and Yorkshire, and the lawless retreats of the border, but was at length discovered, and committed to the Tower. Margaret found refuge in France.
The king was now urged to strengthen his alliances by a marriage with some foreign princess, but he had become attached to Elizabeth Woodville, daughter of the duchess of Bedford, and widow of Sir John Grey, and married her in 1464. The rapid promotions of her family excited great discontent. The powerful earl of Warwick was especially offended. He gave his daughter in marriage to the duke of Clarence, the king's brother, without asking permission ; and after being suspected of aiding insurrections in York