Изображения страниц
PDF
EPUB

on the tops of high mountains than on plains which are but little above the sea-level. Common scales would evidently be inefficient to ascertain this fact, since the weights themselves, as well as the bodies to be weighed, are subject to this diminished influence of gravity. The fact, however, has been carefully ascertained by means of delicate spring balances.

If a force continues to act on any body which is free to obey it, the motion of the body will be accelerated; for the motion generated during the first instant after the force has begun to act, from the inertia of the body, continues afterwards without further force. If, therefore, the action of the force continues, it generates new motion, which is added to the motion of the body during the previous instant, and the speed is thereby continually quickened. We see this exemplified in a falling body, which is thus acted on by gravity. Its gradual descent may be marked by the eye for a time, but, owing to its accelerated motion, it is soon lost except as a mere shadowy line. A ball, dropped from the hand, may be caught after the first instant, but a little delay renders this impossible. A large stone rolling down a mountain side begins its motion slowly, and might at first be stopped or turned aside by any obstacle in its path ; but its velocity is so rapidly increased that it is soon seen bounding from steep to steep, sweeping every obstacle before it. When a body has been falling for any length of time its velocity cannot be measured by the eye, but may be ascertained from its effects. A man leaps from a chair without danger, from a table with a shock, from the top of a house with the fracture of his bones, while in falling from a balloon or down a mine his body is dashed to pieces. When any liquid is poured from a vessel the lower part of the stream moves with a much greater velocity than the part which is just leaving the vessel, but as each part conveys the same quantity of liquid, the stream necessarily tapers towards the bottom. This is well exemplified in pouring out molasses, thick syrup, or oil from a vessel ; and owing to the greater velocity of the lower part of the stream being overlooked, it is a common mistake to imagine that the quantity of fluid is greater at the higher part of the stream than in the small thread which is seen entering the receiving vessel. A body, in falling from a state of rest, acquires a velocity in one second which would carry it through 32 feet during every succeeding second of its fall, without further action of gravity. This velocity is gradually acquired, and therefore its average velocity, during the whole second, is the same as that which it has acquired at the end of the half-second, namely, 16 feet; this, therefore, is the space through which it falls in one second. It falls through the whole 32 feet during the next second, together with 16 feet additional from acceleration, making in all 48 feet, or three times as much as in the first second. In two seconds, therefore, it falls through 64 feet, which is four times the distance through which it falls in one second. At the end of two seconds, the velocity is 64 feet per second, which, with 16 feet additional, gives 80 feet as the space through which it falls during the next second. In three seconds, therefore, it falls nine times as far as in one second. From this law, the velocity of a falling body and the distance fallen through at the end of any given time may be ascertained;—the velocity at any instant is proportional to the time, and the distance proportional to the squares of the time occupied in falling.

While gravity thus acts as a uniformly accelerating force on descending bodies, it has an opposite effect on bodies which are projected upwards from the earth, becoming in this case a uniformly retarding force. The same law which regulates the continually increasing velocity of a falling body, serves in its converse form to explain the gradually diminishing velocity of a rising one. A bullet shot vertically upwards has its velocity lessened every instant until it is brought to a state of rest, when it instantly begins to return, reaching the ground with the same velocity as that with which it started, equal times being occupied in the rise and fall. Since the law of falling bodies applies in its converse form to ascending ones, it follows that if one body be projected upwards with twice the velocity of another, the former will rise four times as far as the latter; if with thrice the velocity, nine times as far, and so on. Hence, it is easy to calculate the height to which any body will ascend, if we know the velocity with which it is projected.

It is said that the genius of Newton read the nature of the law of attraction in the simple incident of observing an apple fall from a tree in his garden. He has beautifully applied it to explain the motions of the heavenly bodies ; and from the lesson he has thus taught us concerning the wisdom of God as manifested in His works, we see a striking instance of the value of observation.

LESSON XXVIII.—WEDNESDAY.

ON THE MASSACRE IN PIEDMONT.
Avenge, O Lord, thy slaughter'd saints, whose bones
Lie scatter'd on the Alpine mountains cold;
Even them who kept thy truth so pure of old,
When all our fathers worshipp'd stocks and stones,
Forget not; in thy book record their groans
Who were thy sheep, and in their ancient fold
Slain by the bloody Piedmontese, that rollid
Mother with infant down the rocks. Their moans
The vales redoubled to the hills, and they
To heaven. Their martyr'd blood and ashes sow
O’er all the Italian fields, where still doth sway
The triple tyrant: that from these may grow
A hundred fold, who having learn'd thy way
Early may fly the Babylonian woe.-Milton.

ON H18 BLINDNESS.
When I consider how my light is spent
Ere half my days, in this dark world and wide,
And that one talent which is death to hide,
Lodged with me useless, though my soul more bent
To serve therewith my Maker, and present
My true account, lest he returning chide ;
“Both God exact day labour, light denied ?”
I fondly ask: but Patience, to prevent
That murmur, soon replies, “ God doth not need
Either man's work, or his own gifts; who best
Bear his mild yoke, they serve him best : his state
Is kingly; thousands at his bidding speed,
And post o'er land and ocean without rest;
They also serve who only stand and wait." - Milton.

Milton! thou should'st be living at this hour;
England hath need of thee :-she is a fen
Of stagnant waters : altar, sword, and pen,
Fireside, the heroic wealth of hall and bower,
Have forfeited their ancient English dower
Of inward happiness.– We are selfish men;
Oh, raise us up! return to us again;
And give us manners, virtue, freedom, power.-
Thy soul was like a star, and dwelt apart:
Thou hadst a voice, whose sound was like the sea;
Pure as the naked heavens,-majestic, free;
So didst thou travel on life's common way
In cheerful godliness : and yet thy heart
The lowliest duties on herself did lay.-Wordsworth.

LESSON XXIX.-THURSDAY.

ENGLISH HISTORY-PLANTAGENET LINE. A short, sad reign, and a violent death, awaited the youthful successor to the throne. Edward V., at his father's decease, was a boy of fourteen years, and was residing at Ludlow Castle, with his maternal uncle, Lord Rivers, under whose guardianship he commenced his journey to London. The duke of Gloucester, brother of the late king, was at this time, with an army, on the borders of Scotland; but he immediately proceeded southward, and met his destined victim at Stony Stratford. Here his first deed of violence was perpetrated. He arrested Lords Rivers and Grey, and despatched them to Pomfret Castle, where they were beheaded soon afterwards. He then dispersed the royal retinue, and accompanying the king to London, assigned the Tower as his residence. Alarmed at these measures, the queen, with her second son the duke of York, and her daughters, fled to sanctuary, from which, however, the young duke was soon removed, and lodged with his brother. Gloucester now assumed the title of Protector of the king and kingdom. The earl of Hastings, who stood in the way of his wicked schemes, was seized and executed, without trial or even accusation. He next attacked the legitimaey of the young princes, to destroy their title to the crown. These methods failing to alienate the loyalty of the nation, bolder plans were adopted.

The duke of Buckingham harangued the people on the claims of the Protector, and afterwards presented to him a paper, purporting to contain a request from the three estates, that he would assume the sovereignty. To this demand Richard, with pretended reluctance, acceded. The unfortunate princes never more left their prison. They were murdered, a very short time after the accession of their unnatural uncle, by his command. The generally received account of their death is, that they were smothered in bed, and afterwards buried at the foot of the staircase that led to their apartment.

The reign of the usurper had hardly commenced when an extensive conspiracy was organized to dethrone him, led by the duke of Buckingham, his late ally, who now espoused the cause of Henry earl of Richmond. The enterprise failed; Buckingham lost his head, and the other chiefs escaped to the Continent, where five hundred English exiles did homage to Henry as their sovereign.

In 1485, Richard obtained from Parliament a recognition of his title to the throne, and proceeded, by effecting some reforms in law, and abolishing, grievances, to diminish his unpopularity, and secure his dominion. But a retribution was at hand. In August of the same year, Henry of Richmond landed at Milford Haven with a hostile force, and, joined by successive reinforcements, advanced to encounter the usurper. The armies met at Bosworth, in Leicestershire, on the 22nd of August, 1485; that of Henry consisting of 6,000 men, his rival's of twice that number. Lord Stanley, with his followers, joined the host of the invader on the eve of the battle, which terminated in the defeat and death of Richard, and the accession to the throne of Henry VII., who was

crowned upon the field. With Richard III. ended the Plantagenet dynasty, which had reigned in England for three centuries and a half; and Henry VII., himself a Lancastrian, by his marriage with the daughter of Edward IV. the heiress of York, united the rival houses, and put an end for ever to the fearful “ Wars of the Roses” which, for more than thirty years, had desolated England.

« ПредыдущаяПродолжить »