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expanse of ocean stretching westward and towards the equator, that they bring the greatest amount of heat, and the most genial influence.
Thus it is that the western coast of Europe is, in the same latitude, greatly warmer than the eastern coast of America, and that there are yet more remarkable differences in the temperature of places having the same latitude, but situated on opposite sides of the latter continent, or on the western and eastern sides of the great continent of Europe and Asia. At New Archangel, on the north-west coast of America (57 deg. 3 min. N. lat.), the mean temperature of the year is 44 deg. of Fahrenheit's thermometer, whilst at Nain, in Labrador, on the same parallel (57 deg. 10 min. N. lat.) of the opposite eastern coast, it is only 27 deg. The mean temperature of the winter in Paris, near the western coast of Europe, is 38 deg., whilst that of Pekin, situated nine degrees nearer to the equator, but on the eastern coast of the continent which comprises Europe and Asia, is only 26 deg.-Moseley.
THE MECHANICAL POWERS.-THE SCREW. If a sheet of paper, of the form of a right-angled triangle, has its perpendicular edge pasted straight along the surface of a cylindrical roller, and the roller is then turned so as to wrap the paper round it, the slant edge of the paper will be seen to wind round the roller exactly like the spiral thread of a screw. This will be best illustrated by making the perpendicular side of the triangular sheet of paper very short in comparison with the length of its base, and drawing a dark ink line along the slant edge on that side of the paper which is seen when folded. Hence it appears that the screw, like the wedge, is a modification of the inclined plane. This is frequently exemplified in the road which leads to the summit of a conical hill being made to wind round the hill, instead of leading directly to the top. A direct road to the summit would be an inclined plane, and a spiral ascent would be the same in principle, although the declivity of the road would be lessened by increasing its length.
In this way any
The spiral of a screw is called the thread, and is made to protrude from the surface of the cylindrical bolt round which it winds. A hollow screw, called a nut, is made to fit the bolt, and for this purpose a spiral groove is cut on its inner surface, the exact counterpart of the thread. The nut is generally turned on the bolt by means of a screw-key or lever, and for every turn it advances along the bolt a space equal to the distance between the threads. In this case the bolt is fixed, but sometimes the nut is fixed, and the bolt is made to advance in a similar manner. resistance that
be offered to the motion of the nut along the bolt, or to the motion of the bolt through the nut, is overcome. The ratio of the power to the weight in this machine is the same as the distance between any two consecutive windings of the thread to the space through which the power is made to revolve. The distance between the threads is generally a very small fraction of the revolving path of the power, and consequently a very small power suffices to move an enormous weight, or exert a great pressure. If a screw have ten threads to an inch, and the length of the lever to which the power is applied be one foot, then a power of twenty pounds will overcome a weight of fifteen thousand pounds. In the practical application of the screw, the power frequently bears a less proportion to the weight than this.
Screws are much used in presses of all kinds. The juices of vegetables, which are employed in medicine or the arts, are squeezed out by means of a screw. The presses of bookbinders and dyers are worked by the same contrivance. Large bales of cotton wool, of which a few comparatively would fill a ship, are readily reduced to compact masses by means of a screw press. The impressions on coins are made by means of a screw, beneath the end of which the die is placed. Machines for punching holes in iron plates frequently derive their enormous pressure from the same contrivance.
A screw can easily be made with a hundred threads to an inch, so that for every turn of the screw it moves through the hundredth part of an inch. But the lever by which the screw is turned has frequently an index attached to it, capable
of marking every hundredth or thousandth part of a turn. If then, a pointed instrument be fixed to the end of the screw, and let down on a metallic plate at every hundredth part of a turn of the lever, it will mark off any number of divisions, each of which would be the ten thousandth part of an inch; and if a mark be made for every thousandth part of a turn of the lever, a hundred thousand divisions would be made in the space of an inch. In this way scales, sextants, protractors, and other instruments are divided, and the screw thus becomes an invaluable piece of mechanism in the hands of the mathematical-instrument maker. On account of its utility in measuring minute portions of space, the screw is also used by the astronomer in adjusting the micrometer for the measurement of the heavenly bodies.
LESSON XVIII.- WEDNESDAY.
penury, can cripple or confine.
ENGLISH HISTORY-PLANTAGENET LINE.
Henry VI., at his father's death, was an infant, only nine months old. He was proclaimed king of France, as well as of England, his dominions includirg the northern and western provinces of that great empire, and Paris, the seat of government, whilst the central and southern parts adhered to the dauphin, afterwards Charles VII. The parliament called in the naine of the infant king appointed the duke of Bedford regent; but, during his absence in France, the office was held by the duke of Gloucester.
The war in France continued, and for several years the English arms were generally victorious. At the battle of Verneuil the French, though assisted by a large body of Scotch troops, were routed, and their allies destroyed. But beneath this apparent success the elements of revolt smouldered. The whole country had been laid wastethe inhabitants crowded into the towns, or betook themselves to inaccessible solitudes—the cultivation of the fields was abandoned — life and property were alike insecure
- famine and the sword depopulated the land. These calamities were all ascribed to the invaders, and as the distress augmented their unpopularity increased.
In 1428, the English laid siege to Orleans, and its surrender seemed certain, when one of the most extraordinary interpositions recorded in history changed the position of affairs, and retrieved the fortunes of France. A peasant girl, of Domremy, in Lorraine, believing herself called to effect the deliverance of her country, presented herself before Charles VII., and obtained from him a body of troops, at whose head she entered the besieged city, with supplies of provisions. Animated by her presence the drooping spirits of the garrison revived, the English were repulsed, and compelled to raise the siege, and depart from Orleans. A similar success attended her at Patay, where Earl Talbot was taken prisoner; and in July, 1429, she accompanied Charles to Rheims, and assisted at his coronation. In the spring of 1430, less than a year after her great triumphs, the heroic Jeanne d'Arc was herself a prisoner in the hands of the English, who, enraged at the disgrace and losses she had caused them, eagerly sought pretexts for revenge.
were ascribed to magic and diabolical aid. She was made to suffer a long imprisonment, aggravated by every infliction that malice and cruelty could suggest; and, after being tried before an ecclesiastical commission, she was burned as a sorceress in the market-place of Rouen. Her death produced to its perpetrators none of the benefits they expected from it. They did not regain their lost possessions, and the chances of war continued unfavourable to them.
In 1431, Henry VI. was crowned at Paris; an empty ceremony, attended with no advantage. In 1135, a congress, held at Arras, was signalized by the death of the duke of Bedford, and the defection from the English party of the