--

SO

from one homogeneous stratum, or mass of stars, that their present isolation, their separation, and various grouping, are only the measured movements of the clock, the gigantic steps of the hand, by which Time records the days of the years of the existing mechanism of the universe? Stupendous the conception, that these great heavens the heavens which we have deemed a synonyme of the Infinite and Eternal are nothing else, after all, than one aspect in which matter is destined to present itself, and that their history is like the birth, life, death, and dissolution, of the fragile plant! If this, indeed, be true, and the idea can be supported by many probabilities, - how immense the sphere of real existence! How little can we ever know of it! at least, how much must be referred to that higher state of existence, an expected eternity of sublime contemplation!

NOTE.-In the preceding pages, under the heads of "Nebular Stars," and "Firmamental Systems," we have given the state of astronomical science, as generally received, at the present time. But some late observations, made by a gigantic telescope, executed for Lord Rosse, seem to render it probable that the diffused, unformed nebulæ, noticed by Herschel and others, are in fact only groups of stars, too remote to be separately distinguished by the telescopes they used. It must now be considered questionable, whether there are in space any masses of matter differing from the solid bodies which compose planetary systems.

MATTER is the general name which has been given to every species of substance, or thing, which is capable of occupying space, or which has the qualities of length, breadth, and thickness; consequently, every thing which can be seen or felt, is said to be matter. In describing the properties of matter, it must be understood that they do not apply to the masses, or substances, commonly met with, but to the uncompounded or primitive materials of which such substances are formed. These original component parts, of which all substances

are made up, are styled simple matter, elementary principles, or, simply, elements. The ancients, as is well known, supposed that there were but four elements, or simple substances-Fire, Air, Earth, and Water; and out of these, or certain combinations of them, all the substances in nature were formed. But modern chemistry, as we shall show hereafter, has discovered that these elements are by no means simple, but capable of being decomposed.

The

Every solid body, or dense mass, possesses what is called a centre of gravity, which is the point upon or about which the body balances itself, and remains in a state of rest, or equilibrium, in any position. centre of gravity may be described as a point in solids which always seeks its lowest level. In round, square, and all regularly-shaped bodies, of uniform density in all their parts, the centre of gravity is the centre of these bodies. When a body is shaped irregularly, the centre of gravity is the point upon which the body will balance itself, and remain in a state of rest.

The line of direction is an ideal line drawn from the centre of gravity of any body, and passing to the ground in a direction perpendicular to the earth's surface. When this line falls within the base of the body, or the part upon which it stands, the body will keep its position; but if the line falls without the base, the body will fall, or overturn. By keeping this principle in view, stability and safety will generally be secured in the erection of works of art,—such as houses, monumental edifices, spires, steeples, as well as in the lading of wagons, and carts, and other vehicles. In every instance, the base ought to be sufficiently large to

admit of the line of direction falling within it. Through ignorance of this principle, and from want of experience, we often see stage-coaches and wagons laden in such a manner that their centre of gravity is liable to too great a change of position, and that they are overturned, to the personal injury, and even loss of life, of the passengers. In the annexed cut, a loaded vehicle

is represented as crossing the side of a hill, which has raised one wheel above the level of the other wheel, so as to incline the body of the vehicle very considerably from the horizontal. The centre of gravity is represented in two different positions; a lower one with the line of direction L C, and a higher one with the line of direction U C. If there had been no load upon the vehicle, the line of direction would have remained at LC; and as it falls within the wheel, or base, the vehicle would have maintained its balance. But if the wagor had been laden, the centre of gravity would have been raised, and, the line of direction U C consequently falling without the wheel, the vehicle must

overturn.

An exception to this rule occurs in the case of XIII.-7

skaters, in making their circular turns on the ice, in which they bend, or lean, greatly beyond the perpendicular position, without falling. This is owing to the contrary effects of centrifugal force, a notice of which will next engage our attention. All bodies, in flying round a centre, have a tendency to proceed in a straight line; and this principle of motion is termed centrifugal force. Examples of this tendency are very familiar to our observation. When we whirl rapidly a string with an apple at one end of it, and suddenly allow the apple to fly off, it proceeds at first in a straight line, but gradually falls to the earth. We see many applications of this principle every day; great use is made of it, also, in manufactures and machinery. In the grinding of corn, and in the making of pottery and glass, it saves much trouble and expense. If a skater or equestrian should stand perfectly upright while turning corners and describing circles, he would inevitably fall on his side, being overturned by the centrifugal force. But by leaning inwards, the centrifugal force is counteracted by gravity, and this forms a support to his overhanging body.

Thus, centrifugal force is the tendency to fly off in a straight line from motion round a centre; and the power which prevents bodies from thus flying off, is called the centripetal, or centre-seeking force. In the case of the apple, the centrifugal force is the impetus given to the apple, which would make it fly away, if the string were to break. The centripetal force is the string, which prevents it from flying away, and gives a circular direction to its motion.

It is upon the mutual action of these two forces that