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MAGNA CHARTA

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MAGNETISM

of instruction rather than of amusement. The picture on the slide should be so sharp as to bear magnifying and so clear as to give a brilliant image. The projection lens should be placed in such a position between the slide and screen that, when the slide is in one conjugate focus, the screen will lie in the other conjugate focus. It is necessary also that this lens be of a rather high grade — corrected both for chromatic and spherical aberration — since the definition of the image on the screen depends immediately upon this lens.

When the screen is viewed from the side on which the lantern is placed, it should be as white and smooth as possible. Since it acts as a diffusing screen, it is essential that no light should be lost by passing through the screen. It should, therefore, be filled with sizing or paint to make it opaque. See Wright's Light.

Magna Charta (m&g'nd kdr'td), the great charter signed by King John of England at Runnymede, June 15, i~*5, has ever since been regarded as the foundation of English liberty. Under the feudal system the tyranny and oppression of the Norman kings had become so great that, in the reign of King John, the English barons rose against him and compelled him to sign this charter. By its provisions a great many abuses connected with the feudal system were abolished. Justice was no longer to be sold or denied to the subject. Life, liberty and property were to be protected against the arbitrary will of the king, and no one was to forfeit either of these except by the law of the land. Fines imposed were to be in proportion to the offense, and even the humblest subject was not to be deprived of his lawful possessions. The great charter was renewed by John's successor, Henry III, in the ninth year of his reign and on five subsequent occasions before his death; and in 1300 it was finally confirmed by Edward I and his parliament.

Magne'sium, a widely distributed metallic element which is never found in the free state. It is present in many minerals, in carbonate of lime and magnesia, asbestos, meerschaum. It exists in mineral waters and in the sea as sulphate and as chloride. A sulphate is Epsom salts, which Drew extracted from the Epsom spring in 1695. The metal was first discovered by Davy. For a long time manufacture was on a small scale; but now it is made in large quantities. Chlorides of oo-tassium and magnesium and fluorspar, with metallic sodium added, are fused together. The crude metal is finally distilled and pressed in a semifluid state into ribbon or wire. The metal is also prepared by passing an electric current through a fused mixture of salts instead of using metallic sodium. It has a silver-white color, which is tarnished by moist air. It is very light, readily volatile, and, when lighted, burns in air with an intensely brilliant light rich in chemical rays.

On this account it was much used in photography until the electric light took its place. The medicine calcined magnesia is the oxide of magnesium, the same substance as that formed when magnesium is burnt.

Mag'netism. It has been known for many centuries that an iron ore which mineralogists call loadstone or magnetite has the remarkable property of attracting iron filings. A body which possesses this property is said to be magnetizedj and is called a magnet. Cer-

MAGNET  LIFTING   GENERATOR WEIGHING 8oO POUNDS

tain parts of a magnetized body attract iron-filings more strongly than do others. These parts are called magnetic poles. A piece of iron can be magnetized by rubbing it over a piece of lodestone; and if the iron have a long, slender shape it will ordinarily have only two poles. The first important discovery in magnetism was made some time near the 12th century of the Christian era, when it was found that if a piece of magnetized iron be freely suspended it always sets itself so that a certain direction in it makes a fixed angle with the geographical meridian, i. e., with the north-and-south line. This is the fundamental principle of the ordinary mariner's compass.

The direction in which the freely-suspended compass-needle points, at any place, is called the magnetic meridian at that place. The angle between the magnetic and geographical meridians is called the magnetic declination.

Before the time of Columbus' first voyage