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linear force of the projectile was equal or inferior to the attractive force of the earth at their first meeting together.

Yet this is, perhaps, but little more than the velocity with which a twenty-four pound cannonball would travel from the moon's surface: since its velocity on the earth's surface often amounts to 2000 feet per second; and it would rush much more rapidly if not impeded by the resistance of the atmosphere. And hence it is to this cause that M. Olbers first, and M. Laplace has since, ascribed the origin of those wonderful aërolites, or stones, that are now known to have fallen from the air at some period or other in every quarter of the globe; believing them to be in every instance volcanic productions of the moon, thrown by the impulse of the explosion beyond the range of her centripetal influence.

COHESIBILITY is the tendency which one part of matter evinces to unite with another part of matter so as to form out of different particles or portions one common mass. It includes the three modes which have often been regarded as three distinct properties, of extension, density, and impenetrability. EXTENSION is a term as applicable to space as to matter: "The extension of body," observes Mr. Locke, "being nothing but the cohesion or continuity of solid, separable, movable parts; and the extension of space, the continuity of unsolid, inseparable and immovable parts." Hence extension applies to all directions of matter, for its continuity may take place in all directions; but in common language the longest extension of a body is called its length, the next its breadth, and the shortest its thickness.

DENSITY is that property of bodies by which they contain a certain quantity of matter under a certain bulk of magnitude. Thus, a body, whose particles are more compactly arranged, or contains more matter than another, though of the same bulk, is said to be more dense than the other. Hence density cannot be a property of space, the parts of which, as I have just observed, are immovable, and cannot, therefore, either approach or recede, or be more or less close.

In

IMPENETRABILITY is that property in matter which prevents two bodies from occupying the same place at the same time. They are all branches of the common property of cohesibility. A wedge of iron, we know, may force its way through the solid fibres of the trunk of a tree; but it can only do this by separating them from each other: it cannot penetrate the matter of which those fibres consist. like manner, when a ship is launched, her hulk cannot sink into the water without displacing the exact bulk of water which existed in the space that the hulk below the surface now occupies. And so, again, it may be shown by easy experiments, that even air, and the gases manifest their impenetrability as decidedly as solid bodies.

To a cursory survey, however, there are some phænomena that seem to show that certain bodies are penetrable by others. Thus, if a cubic inch of water be mixed with a cubic inch of spirit of wine or sulphuric acid, the bulk of the compound will be something less than two cubic inches. But in this case one of the fluids appears to admit a part of the other fluid into its pores, not into its substance; a fact of which there can be little doubt, since, if no

evaporation be allowed to take place, though the bulk of the mixture is somewhat diminished, its weight is precisely equal to what it ought to be. The combination of different metals affords, not unfrequently, similar instances of equal introsusception; yet equally reconcileable with the general doctrine of impenetrability.

DIVISIBILITY is a power in matter directly opposed to its cohesibility. It is that property of a body by which it is capable of separation into parts, the union or continuity of which constituted its

extension.

Divisibility, however, does not destroy cohesion in every instance equally; though the farther it proceeds, the farther it loosens it. We are told by Mr. Boyle, that two grains and a half of silk were, on one occasion, spun into a thread not less than three hundred yards long, which is, notwithstanding, a much shorter length than the spider is capable of spinning his web of the same weight. Muschenbroek mentions an artist of Nuremburg, who drew gold wire so fine that 500 inches of it only weighed one grain ; and Dr. Wollaston has obtained platinum wire as fine as 30th of an inch.* The thickness of tinfoil is about a thousandth part of an inch+; that of gold-leaf is less than a two hundred thousandth part of an inch; and the gilding of lace is still thinner, probably in some cases not more than a millionth part of an inch; and there are living beings, visible to the microscope, of which a million million would not make up the bulk of a common grain of sand.

* Wollaston in Phil. Trans. for 1813, p. 114. Annals of Philos. No. III. p. 224.

Davy's Elem. vol. i. p. 379.

Thomson's

Yet it is highly probable, from what has actually been ascertained of the anatomy of minute and microscopic animals, that many of these are as complicated in their structure as the elephant or the whale. How exceedingly minute, then, must their vessels be, and how much more so the particles of which they are constituted, and the particles of the fluids which flow through them! And how infinite, as to the intellectual conception, the divisibility which here exists!

GRAVITY is the bond which connects all the bodies in the universe, and of whose existence and agency we have daily experience. Gravity, or, as it is usually expressed by the verbal noun, Gravitation, is the attraction by which bodies of all kinds act upon each other, with a force regulated by the aggregate proportion of their respective quantities of matter, and decreasing as the squares of the distances increase. It is a principle residing in matter universally, and operating alike on the minutest and on the largest masses; producing what we call weight on earth, or the tendency of heavy bodies to fall towards the earth's centre. This principle governs the revolutions of the planets. The five principles which regulate its mode of action, and constitute its magnificent code of laws, are thus summed up by M. Laplace:*—

1. Gravitation takes place between the most minute particles of bodies.

2. It is proportional to their masses.

3. It is inversely as the squares of the distances. 4. It is transmitted instantaneously from one body to another.

* Exposition du Système du Monde.

5. It acts equally on bodies in a state of rest, and upon those which, moving within its range, seem to be receding from its power.

To a casual observer there are many substances that seem to fly away from the earth, and consequently to oppose this general law. Thus smoke, when extricated from burning bodies, and vapour, when separated from liquids, ascend into the atmosphere; and a piece of cork, plunged to the bottom of a vessel of water, rises rapidly to the surface. But, in all these phænomena, the bodies that seem to move upwards merely give way to bodies of a heavier kind, or, in other words, which have a stronger tendency towards the earth. Thus smoke and vapour only ascend, because the surrounding air, which is heavier than these, presses downwards and takes their place; and the cork rises because lighter than the water into which it has been plunged: but empty the vessel, and the cork will remain at the bottom, because heavier than the surrounding air; and let the smoke or the vapour be received into a vacuum, and it will remain as much at the bottom as the cork.

It was first demonstrated completely and satisfactorily by Sir Isaac Newton, that all the heavenly bodies in their motions obey the same power, the orbit of each being in consequence a conic section; and the principle thus struck out has of later years been still more extensively and even more accurately applied to a solution of the most complicated phanomena. This general attraction is in astronomy denominated the centripetal force, and the term is sufficiently precise for all common purposes; since, although, speaking with perfect strictness, the cen

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