USE OF THE GLOBES.*

THE terrestrial globe is a representation of the earth, with the sea and land on its surface, and the several circles necessary for determining their positions. The celestial globe is a like representation of the stars. Each

globe is suspended, by means of an axis, in a brazen ring, called the universal or brazen meridian, and supported in a frame, the upper part of which is flat, and represents the rational horizon.

The universal meridian is divided into degrees, and parts of degrees: and it is numbered on one side from 0 at the equator, both ways, to 90° at the poles; and on the other side from 0 at each pole, to 90° at the equator.

On the terrestrial globe the equator is numbered, both eastward and westward, from the point in which it is cut by the first meridian. It is also divided into twenty-four equal parts, corresponding to the hours of the day.

Each globe is furnished with a small circle of brass, called the hour circle. This is placed at the north pole, and is divided into twenty-four equal parts, to represent the hours of the day.†

The horizon of each globe is divided into degrees; and is numbered by one series of figures, commencing from 0 at the north and south points, and ascending to 90° at the east and west; and by another series, commencing at the east and west points, and terminating at the north and south. The several points of the compass, the months of the year, and the signs and degrees of the ecliptic in

*The following article will contain merely the most important of the problems that can be solved by means of globes; many being omitted which are usually given in treatises on the subject, but which avery intelligent teacher will perhaps consider to be much too numerous, and, in many cases, too little elementary, to be intelligible or useful to the generality of pupils.

On some globes, the hour circle is fixed, and has a moveable index. On others, however, the circle is moveable, and the meridian serves instead of an index. This mode is much preferable, as the index is very liable to go out of order. Some globes have another circle at the south pole. When globes differ in this or in other respects from the description here given, the pupil will in general feel no difficulty, if he consider carefully what is here stated.

which the sun is on each day, are also marked on the horizon.

The quadrant, or the quadrant of altitude, is a thin slip of brass, numbered from 0 to 90° in one direction, and from 0 to 18° in the other.

PROBLEMS ON THE GLOBES.

PROBLEM I.-To find the latitude and longitude of a given place.

Rule.-1. Bring the place to the universal meridian.. 2. Then the degree of the meridian above the place is its latitude; and, 3. The degree of the equator cut by the meridian is its longitude.*

Required the latitudes and longitudes of the following places:

Exercise 1. Moscow Ex. 3. Pekin

Ex. 5. Belfast 6. Batavia.

PROBLEM II.-Given the latitude and longitude of a place; to find it on the globe.

Rule.-1. Bring the given longitude found on the equator, to the universal meridian. 2. Find the given latitude on the meridian, and the point below it will be the required place.+ ·

Find the places whose latitudes and longitudes are as follows:

Ex. 7. 16° S. 530 W. Ex. 8. 56° N. 3° W. Ex. 9. 2240 N. 88° E.

Thus, the latitude of Palermo is 381° north, and its longitude 131° east. If two places lie on the same side of the equator, their difference of latitude will be found by subtracting the latitude of the one from that of the other; but if they be on opposite sides, the latitudes must be added. To find the difference of longitude of two places, add their longitudes if one be east and the other west; otherwise, subtract. If the result obtained by adding should exceed 180°, subtract it from 360°, and the remainder will be the required difference.

+ Thus, the place whose longitude is 781° west, and latitude 333° south, will be found to be the island of Juan Fernandez.

by mutual attraction, so as to form a dependent and con nected group.

UNIVERSAL ATTRACTION.

EVERY portion of matter with which we are acquainted is attracted by every other, or has a tendency to move towards it. Mountains attract plummets placed near them, and prevent them from hanging perpendicularly.* The Earth attracts the Moon, the Moon the Earth, the Sun the planets, and the planets the Sun and one another. All bodies at the Earth's surface tend to move towards its centre, and this tendency constitutes their weight. This arises, not from any attractive power lodged in the centre more than in other parts of the Earth, but because a great mass of matter lies in that direction, without any in the opposite to counteract its effect; while, whatever lies on any side of the line passing through the centre, is prevented from causing any motion towards itself by an equal mass placed on the opposite side, so that the joint effect of both is still to cause bodies to have a tendency towards the centre.

All the motions of the bodies in the solar system, are produced by a due combination and adjustment of such an attraction, and of a projectile force originally impressed upon each of them. Thus, a planet is attracted towards the Sun by a force, which, were its effect not counteracted, would cause it to fall to his body. It has also a motion in its orbit, which, if the Sun's attraction were suspended, would carry it forward with a constant velo city, in a straight line touching the orbit. Instead of thus receding, however, into infinite space, it is per

*The magnitude of the Earth so vastly exceeds that of the largest mountain, that a plumb-line in the circumstances above referred to, deviates from the perpendicular only by a few seconds. The moun tain of Schihallion in Scotland, by experiments made in 1774, under the direction of the Royal Society of London, caused a deviation of 5.8. From calculations founded on this result, compared with the Earth's attraction, and on the magnitude and density of this moun tain, it has been inferred that the mean density of the whole Earth is nearly five times as great as that of pure water.

vn off, by the Sun's attraction, from the th, and caused to describe an elliptical

elocity in the orbit much greater than it is, the attracbe insufficient to cause the body to move in an ellipse. ould describe a parabola or hyperbola; and thus rely from the Sun, and losing the benefit of his inin all probability become unfit for both animal and If, on the other hand, the velocity were much less inet would be brought so near the centre of the sysfall on some part of the Sun's body, or at least to be fluence in such a degree as to render it unfit to be a mated beings. Even with the medium velocity, also, ion of the motion were nearly perpendicular, as it is he line drawn from the planet to the Sun, the orlit pse of great eccentricity, and the planet would be the inconveniences arising from such an orbit. It emarked, that the attractive force is found to be innal to the square of the distance; and it can be shown that were the law of variation of the force different anetary orbits could not retain their present forms . Nothing, therefore, can afford a stronger proof of ligence in the structure of the universe, than the accuof these three elements, the law of the force, the veirection, which all admit of infinite varieties; and a nge in any of which, in relation to a particular planet, the comfort, and perhaps even to the very existence,

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al principles which have been here alluded to, it may ferred, that the fixed stars must be in motion as well -an opinion which also seems to be confirmed by obe appearances seem to indicate motions among them, may require many ages to make them sensible to us, of the vast distances of the stars, may yet be extremely ed stars, however remote, may be expected to attract d thus, though the effect may be excessively small in their extreme distance, its ultimate tendency would cause them all to be collected into one vast mass, unthe attractive influence were counteracted by a propressed on each. Hence, each nebula may be a ed of myriads of stars, completing their revolutions non centre of gravity of the nebula itself, in periods of he ages that have passed since the creation of our pla1 but an inconsiderable atom; and many of these vast ain be combined, to form another of still ampler and ble dimensions. According to this view, the stars of e probably distributed into numerous vast systems, s at such a distance from the rest, that, viewed from

THE TIDES.

THE tides are alternate elevations and depressions of the waters of the sea, which take place twice in the lunar day, or in the average space of about 24 hours 50 minutes These are occasioned by the attractions of the Sun and Moon. Thus, while the Earth is performing its revolu tion, it is drawn by the Sun from the rectilineal path, in such a manner, that its centre describes its orbit. At the same time, the side nearest the Sun, being attracted with greater force, is drawn farther from the rectilineal path than the centre; while the most remote side is drawn less than the centre from the line in which it would otherwise move; and thus the distance of each from the centre, is increased.* Hence it appears, that,

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it, they appear, till examined with telescopes, to be merely nebulas; while the stars which are distinctly visible, all belong to the system in which the observer is placed. Dr. Herschel supposed, and parently with reason, that our Sun and system belong to a vast rebula, which extends around us to an immeasurable distance, and forms what is called the Milky Way. The milky way, or galaxy, is a broad track extending round the sky, and distinguishable by its whitish ap pearance. Its course lies through the constellations of Cassiopeia, Cygnus, Perseus, Andromeda, Gemini, and several others. If Dr. Herschel's opinion, above alluded to, be true, it is a curious analogy, that the numberless stars in this collection should be so placed as to form a zone of limited breadth, in the same manner in which the or bits of the planets in the solar system all lie in nearly the same plane. Farther information on these interesting subjects will be found in Paley's Natural Theology, in several papers by Dr. Herschel, in the Philosophical Transactions, and in various works on Astronomy.

* This part of the theory of the tides, in which students generally feel some difficulty, will perhaps be rendered more simple, if we sup pose a planet with two satellites of equal masses and at equal distances, revolving round the Sun in such a manner that one of the satellites is always between the Sun and planet, and the other in opposition to the Sun. In these circumstances, it is evident, that the primary planet would describe its orbit just as if it were attended by no satellites, and that, from the inequalities of the attractions exerted on it and on the secondaries, the distances between it and each of them would be increased. The theory may perhaps be rendered still farther intelligible by considering, that the centrifugal forces of the two satellites would be, one of them less and the other greater than that of the primary, in consequence of the one moving with a less and the other with a