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and invented that beautiful planetary system, whith is universally rectivel. He developed, wiih mathematical precision, all the phenomena of light and colders, the nao ture of vision, and the use of optical glasses and instrum ments, which last, he greatly improved. To conclude, he gave body and cousistency to natural philosophy, and made it what it deper was before, a system of trath, illustrated and proved by experiment.
CHAP. I.-MECHANICS MECHANICS treats of the laws of the equilibrium and motion of solid bodies; of the forces by which animate or inanimate bodies may be made to act upon one another; aud of the mans by which these may be increased. It is usually syldivided by scientific wriers into Stuties and Dynumics, the former relating to the mutual action of bodies at rest, the latter to the action of bodies in diotion. Hence, under, stutics, is comprehended the doctrines of simple machines iu a state of equilibrium, the properties of the centre of gravity, the strength and stress of various substances, the pressure of arches and domes, the stability of piors, and the scientific construction of roofs and centering: while underdynamics, philosophers treat of rectilinear, rotatory, and orbitual motion, the oscillation of pendulums, the force of gy rating or revolving bodies, the motion of bodies exposed to the action of centripetal or centrifugal forces, and the maximum effects of machines: 1. The science of mechanics is dependent upon the power which gravity and the laws of motion-have on matter. The arioms, or laws of motion and rest, as first laid down by Sir Isaac Newton, are: 1. Every body perseveres in a state of rest, or of unāform rectilinear motion, until a change is effected by some external cause. : 2. Every motion, and every change of motion, is proportional to the force impressed and in its direction. 3. Action and reaction are equal and contrary. . These laws, though they are called axioms, are by no means self-evident. The best illustrations and contimations of them we remember to have seen, are given by Mr. Atwood in his treatise on Motion, and by Dr. GREGORY' in the first volume of bis comprehensive treatise of Mechanics. Mutter comprizes whatever is extended and
capable of makiug resistance; hence because all bodies, whether solid or Auid, are extended, and do resist, we conclude that they are constituied of matter. Every thing wbich is the object of our senses, is composed of matter differently modified. Thus, for instance, water rarefied by heat, beçoines vapour; great collections of vapours form ciouds; these condensed, descend into the form of dail or rain; part of this collected on the earth constitutes rivers; another part mixing with the earth eaters into the roots of plants, and supplies matter to, and expands itself into various species of vegetables.
2. It is very probable that all the variety we observe in matter, arises from the various forms and shapes it puts on. Sir Isaac Newton is of opision, that God is the beginning formed matter into solid, massy, impenetrable, moveable particles or atoms of such sizes and figures, and with such other properties, and in such proportion to space, as most conduced to the end for which he formed them; and that these primitive particles being solids, are incomparably harder than any porous bodies compounded of them, even so hard as never to wear or break in pieces; no ordinary power being able to divide what God himself made one in the first creation. The essential properties of matter, are solidity, divisibility, mobility, and inertia, or a state of inaction.
3. The solidity of matter is not here considered as opposed to fluidity, but as that property which every body possessess of not permitting any other substance to occupy the same place with it at the same time; so that both water and air, and every other fuid are equally solid, in this sense with the hardest body. Solidity, in this sense, is equivalent to impenetrability. A tube filled with water will not admit a plunger accurately fitted to it, unless the water is first displaced. The solidity of matter is manifest by the resistance it makes to the touch. By divisibility, the particles of all bodies are capable of a separation, or dişunion from each other. The atoms of which bodies are formed, are concealed from us by their minuteness, and though they are the active parts of matter, we can form NO idea of them, for whether we view animate or inanimate matter, the corpuscles of which it is formed, are so infinitely small, as to escape the scratioy' of the highest magnifying powers. The infinite, or at least indefinite divisibility of matter is forcibly shown by the different perfumes exhaled by plants and flowers. How surprisingly minute must the odoriferous corpuscles of the carnation be which diffuse theinselves over a whole garden, and every where meet the sense of smell. A grain of gold can be beaten into a leaf containing fifty square inches, and this leaf may be readily divided into 500,000 parts, each of which is visible to the naked eye. Lewenhoek discovered more living animalcula in the milt' of one single cod-fish, than there are animals on the earth ! If we suppose these animalcula to be furnished with blood, like other animals, and if the globules of their blood bear the same proportion to their bulk as those of a man bear to his body, it may be proved, that the smallest visible grain of sand would contain more of these globules than 10,256 of the largest mountains in the world would contain grains of sand. Nevertheless, if these particies were compared with the particles of light, it is probable they would be found to exceed them in bulk, as much as mountains do single grains of sand. In pepper put into a glass of water, animalcula have been discovered, a thousand millions of times less than a grain of sand.
4. Mobility is that property of matter, by which it can be moved from one place to another. No particle of matter can give to itself either motion or rest; and, therefore, according to the first law, a body at rest will reinain so for ever, except some external cause puts it in motion'; and a body in motion will continue to move, except prevented by some external force. A body in motion will always move on in a straight line, unless it be turned out of it by some extemal cause ; because a body can no more alter the determination of its motion than it can begin it, alter, or stop its motion itself. · All material objects are of themselves without motion, yet by the application of a proper force any body may be moved. A howl impelled along the ground, or a cannon ball going through the air, passes froin motion to a state of rest, by the friction of the earth; their own weight; or by resistance from the air. The momentum, or quantity of notion, in a moving body is represented by the product of its quantity of matter or mass into its velocity. So, if a body which weighs 4 pounds move with
a velocity of 15 feet per second, and one which weighs 3 pounds, with a velocity of 20 feet per second, they would move with equal momenta, and would require equal momenta, either to stop them, or to give them their respective motions. By inertia, bodies persist in a state of motion or rest, receive motion in proportion to the force impressing them, and resist as much as they are resisted. It is nothing more than the power of resistance, through which all bodies are disposed to remain in that state in which they are. . If the celestial bodies had not this power of resista ance, they could not move with so much order and regularity, and must have continually a new moving cause to preserve them in motion.
5. The attraction of cohesion is that power by which the small particles of matter are kept together. By this principle, bodies preserve their forms, and are preserved from falling in pieces; and it is probably owing to the various degrees of cohesion that some bodies are solid and others fluid. Cohesion and adhesion are now generally distinguished from each other. Cohesion is the force which unites particles of the same body together; adhesion is that which is found to dispose different or separated bodies to unite at their surfaces. Two leaden balls having each a smooth surface, if strongly compressed together, will adhere almost as strongly as if united by fusion; and this is nearly observed in two plates of glass. Two globules of quicksilver, when brought near each other, will run together, and assume the same appearance. By the attraction of cohesion are formed stones, metals, woods, salts, and every thing that may be denominated a body. Petrifactions, porcelain, pottery, bricks, glass, cements, artificial stones, and plastic earthy compositions, which preserve their figure in drying, are all children of this great agent; and as this power is much greater in some bodies than in others, there arises an infinite variety in the strength, weight, and texture of metals. It is upon this principle also, that carpenters and cabinet-makers use glues: that braziers, tininen, and plumbers solder their metals, and that smiths unite different bars of iron by the agency of heat. The attraction of gravitation is one of the most universal principles in nature. It is that force by which distant
bodies tend towards each other. By gravity, a stone dropped from a height falls to the earth; by it the heavenly bodies are kept in their orbits; and hereby all terrestrial bodies tend towards the centre of the earth. By it, the planete tend towards the sun, and towards each other, as well as the sun does to thein. From this attraction arises all the motion, and consequently all the changes that take place, in the grander portions of the universe. By this, heavy bodies descend, and light ones ascend; by this, projectiles are directed, and rains fall; rivers glide, the octan swells, and the air presses upon different bodies. By capillary attraction, water or other liquids will ascend in sugar, sponge, and all porous bodies. It is thus denomiuated, from the property wbich tubes (scarcely capable of admitting a hair) have, of causing water to stand above its level.
6. The centre of gravity of a body is that point about which all the parts of a body, in any situation, balance each other. Hence, if a body is suspended or supported by this point, the body will rest in any position in which it is put. Also, whatever supports that point bears the weight of the whole body : aud while it is supported, the body cannot fall. We may therefore consider the whole weight of a body as centred in this point. If a line is drawn froin the centre of gravity of a body, perpendicular to the horizou, it is called the line of direction ; because it is the line that the centre of gravity would describe, if the body fell freely. The broader the base, and the nearer the line of direction is ío the middle or Centre, the more firmly does the body stand. On the contrary, the pare rower the base, and the nearer the line of direction is 10 the side of it, the more easily may the body be overthrown. Every body in motion must have a deterininate velocity, and it is estimated by the space passed over unifornly by the centre of gravity in time. Acceler.ited motion is that in which the velocity is continually increasing. When its velocity is continually decreasing, it is said to be returded. If the augmentations of motion are equal in equal times, the motion is said to be uniformly acceleratret; if equal diminutions take place in equal times the motion is uniformly returded. The former is exemplified in the case of