objects, the distance of which is too great to be observed by the micrometer.

THE TRANSIT INSTRUMENT.

This instrument is used for observing objects as they pass over the meridian. It consists of a telescope, fixed at right angles to a horizontal axis, which axis must be so supported that the line of sight of the telescope may move in the plane of the meridian.

THE MICROMETER.

The micrometer is an instrument, by which small angles, or the apparent magnitudes of objects viewed through telescopes or microscopes, are measured with great exactness. It effects this end by pieces of metal ground to a very fine edge; or fine hairs; or a net of silver wire, applied to the object glass of a telescope; or by specula; or by the light of a lamp.

THE TELESCOPE.

The telescope is an instrument that enlarges the visual angle subtended by a distant object, and thereby is said to magnify it, so as to render it visible to the eye of an observer. Thus it makes distant objects appear close to the eye.

The effect of the telescope depends upon this simple principle, namely, that objects appear larger in proportion to the angles which they subtend at the eye; and the result is the same, whether the pencils of rays by which objects be

come visible to us come directly from the objects themselves, or from any place nearer to the eye, where they may have been converged, or brought together in a point, so as so form an image of the object. In fact, therefore, all that is effected by a telescope is, first, to make such an image of a distant object, by means of a lens, that is, a glass ground in a particular manner, or by a mirror; and then to give the eye some assistance for viewing that image as near as possible. This is done by means of an eye-glass, which so refracts the pencils of rays that they may afterwards be brought to their several foci by the humours of the eye. Such is the telescope which was first discovered and used by philosophers. The rays of light proceeding from the object itself, passing through glasses, were made to subtend larger angles at the eye than they would have done without such aid; and thus the object was magnified, and apparently brought nearer to the beholder. This effect of two lenses, or glasses, one concave and the other convex, placed at a proper distance from each other, appears to have been discovered by chance, towards the end of the sixteenth century, This discovery is attributed to a philosopher named Metius; to Lippershum, a spectacle maker at Middleburg; and to Jansen, another optician of the same place, A.D. 1590. Whichsoever of them first remarked the property that such a combination of glasses possesses, of showing distant objects, placed the glasses in tubes, and thus made the first telescope. The term telescope is combined from two Greek words, signifying distance and

the action of seeing. For many years after this discovery, no other telescope was imagined. Galileo and others improved it, and, by its assistance, made important discoveries concerning the heavenly bodies. These were called refracting telescopes, and they were obliged to be made of an unmanageable length, in order to increase their powers to any very great degree. To remedy this, was invented the reflecting telescope, which magnifies the reflected images of objects, and not the objects themselves. For the eyeglass of the common telescope, is substituted a metallic speculum or mirror of a parabolic figure, to receive the rays coming direct from the object observed, and to reflect them towards a smaller speculum of the same metal: this returns the image to an eye-glass placed behind the greater speculum, which, for that purpose, is perforated in the centre. This invention is attributed to Gregory of Aberdeen, and was published in 1663; but Sir Isaac Newton, and others since his time, have greatly improved the reflecting telescope; and Herschel seems to have brought to perfection this instrument, which has so wonderfully extended the sphere of human vision.

QUESTIONS.

What is the orrery, and whence does that instrument derive its name? What is the instrument called planetarium? What is the cometarium? The trajectorium lunare? The pendulum clock? What is the mural quadrant, and the portable quadrant? sector? The transit instrument? whom, and when, was the telescope

What is the equatorial The micrometer? By invented? What does

that instrument effect? What is the construction of the refracting telescope? What is the nature of the reflecting telescope?

CHAP. XXIX.

NAVIGATION.

NAVIGATION is the art of conducting a ship from one place to another, and of steering its course over the waters of the ocean, in the safest, shortest, and most commodious way.

Historians represent this art to have been first cultivated by the Phoenicians; especially by the inhabitants of Tyre. Mount Lebanon and the neighbouring hills furnished them with abundance of excellent timber; of which taking advantage, they built numerous ships, and sent their merchant fleets, not only round the shores of the Mediterranean, but also through the straits of Gibralter into the vast Atlantic ocean, along the western coasts of Africa and Europe. The Carthagenians, Phoenician colonists, ímbibed the spirit of the mother country, and paid particular attention to navigation. By means of their superior skill in this art, they long maintained successful opposition to the Romans, and attained, through commerce, to great wealth and power. Hanno, one of their admirals, is said to have circumnavigated Africa. From Phoenicia, upon the destruction of Tyre by Alexander the Great, the science of Navigation was transferred to Alexandria in Egypt, a city built by that

commerce.

mighty conqueror, for the express purpose of serving as an emporium for eastern and western The Greeks, likewise, had, before that period, paid great attention to navigation, and had settled colonies on different parts of the coasts of the Mediterranean, and the Black sea. The fall of the Roman empire for a time extinguished this as well as the other sciences and

arts.

Navigation revived in Italy, where the Venetians and the Genoese cultivated it to a high degree, to their own great profit. But till the invention of the compass, and its application to the purposes of navigation, in the year 1420, it must have been little better than mere coasting; that is, directing the course of vessels by headlands, promontories, and capes, and by observations made upon certain of the fixed stars. This is called navigation common.

But navigation has been vastly improved in modern times, both with regard to the form of the vessels, and with regard to the methods of working them, The use of oars is now almost entirely superseded by the vast improvements made in the formation of sails and rigging; so that ships not only sail much faster than in ancient times, but can also tack, or be turned, in every different direction. The adventurous mariner now finds his way across the trackless ocean to the most distant regions of the globe, and can determine exactly the path his vessel takes, and what are her successive situations with respect to other parts of the earth's surface. This art connects the remotest countries and nations ;