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constant velocity. Thus the general doctrine of forces has come to be considered in two points of view, according as they balance each other in a state of rest or motion. These two ways of considering the same subject require different principles, and a different mode of reasoning. The first has been named Statics, as expressing that rest which is the test of this kind of equilibrium. The second has been denominated Dynamics, or Universal Mechanics, because the different kinds of motion are characteristic of the powers or forces which produce them. The science of statics therefore, is preparatory to the study of mechanics in this enlarged view of it, and is the foundation of many useful parts of knowledge, which have been thus enumerated. 1. It comprehends the doctrine of the excitement and propagation of pressure, by which the energies of machines are produced. A pressure, for instance, is exerted on the impelled point of a machine, such as the float-boards or buckets of a mill-wheel. excites a pressure at the pivots of its axle, which acts on the points of support: a pressure is also excited at the acting-tooth of the cog-wheel, on the same axle, by which it urges round another wheel, and so on to others, and by these means a pressure is ultimately excited in the working point of a machine. The science of statics teaches how to find the intensities and directions of all these pressures, and how much remains at the working point of the machine unbalanced by resistance. 2. It comprehends every circumstance which influences the stability of heavy bodies; the investigation and properties of the centre of gravity; the theory of the construction of arches, vaults, and domes; and the attitudes of animals. 3. The strength of materials, and the principles of construction, so as to make the proper adjustment of strength, to the supposed strain, in every part of the machine, &c. In practice, therefore, statics furnish what may be called a theory of carpentry, and affords instructions for framing floors, roofs, centres, &c. 4. This branch of science comprehends the whole doctrine of the pressure of fluids, whether liquid or æriform, whether arising

from their weight, or from any external action. Hence is derived a knowledge of the stability of ships, and their power of maintaining themselves in a steady and upright position, in opposition to the action of the wind on their sails. On these and other topics of the like kind, the reader may be referred to "A Treatise of Mechanics, Theoretical, Practical, and Descriptive, by Olinthus Gregory, LL. D." in 2 vols. 8vo. The first contains the theory of Statics, Dynamics, Hydrostatics, Hydrodynamics and Pneumatics; and the second contains Remarks on the Nature, Construction, and Simplification of Machinery; on Friction, the Rigidity of Cords, First Movers, &c. with descriptions of many curious and useful Machines. To this we shall add "A Treatise on Rectilinear Motion and the Rotation of Bodies, &c. By G. Atwood, F. R. S." which contains a variety of topics that will claim the attention of those who have advanced beyond the mere elements of this branch of science.

"Mathematical Magic, or the Wonders that may be performed by Mechanical Geometry, &c. by John Wilkins, late bishop of Chester."

The author of this entertaining work, informs us that the reason of the title, Mathematical Magic, was because the art of such mechanical inventions as he has described, had been usually attributed to the power of magic. The first book is entitled Archimedes, in honour of him, to whom is to be chiefly assigned the invention of the mechanical powers. The second, he styled Dadalus, after him, who is distinguished among the ancients for his skill in making automata, or selfmoving engines, and for reducing to practice the mathematical principles of philosophy.

The enumeration of the various contents of these books would require a larger space of our work, than can be allowed to a single treatise; it will be sufficient to inform the reader that they are all interesting, and cannot fail to afford real entertainment to persons who have a taste for subjects of this kind.

CHAP. III.

NATURAL PHILOSOPHY,

Continued.

OPTICS, principles of-Nature of light-Refraction of the rays of lightReflexion of light-Different refrangibility of the rays of light-The Rainbow-Vision and structure of the Eye-Optical InstrumentsMicroscopes Telescopes-Camera Obscura-Magic Lantern-Phantasmagoria-Writers on Optics.

THE science of Opties so important to the purposes of life, is a mixed mathematical science, which explains the manner in which vision is performed in the eye; it treats of sight in general; gives the reasons of the several modifications or alterations which the rays of light undergo in the eye; and shews why objects appear, under different circumstances, of different magnitudes, sometimes more distinct, sometimes confused, sometimes nearer, and sometimes more remote. In this signification, the science of Optics is considered by Sir Isaac Newton. The history of the science has been detailed by the illustrious Priestley, in a large quarto volume, which has generally been considered as one of the most interesting of his numerous works. In a small compass, such as the nature of this work would admit, it would be scarcely possible to include a sketch even, of the history, that would be intelligible. We

VOL. II.

shall, therefore, proceed to explain some of the fundamental principles of the science, such as the nature of light, the laws of refraction and reflexion, the nature of vision, and the structure of some of the commoner and more useful instruments.

Of the Nature of Light. It is generally agreed, though the subject does not admit of demonstration, that light consists of inconceivably small particles, flowing with amazing velocity, in all directions, from the luminous or radiant body. This theory of light appears the most simple of any, and serves to explain all the phenomena of vision; and, therefore, has by the majority of writers on the subject, been assumed as

true.

The velocity with which light moves, was first observed by M. Roemer, who ascertained that it travelled from the sun to the earth, a space of 95,000,000 of miles, in about eight minutes, that is at the rate of about 200,000 miles in a second of time. This fact wås inferred from the following circumstance: the eclipses of Jupiter's satellites, happen sometimes sooner and sometimes later than the times given by the tables, according as the earth is nearer to, or farther from that planet. Thus, when the earth is at C, Pl. III. fig. 1, between the sun and Jupiter, his satellites are seen eclipsed about eight minutes sooner than they would be, according to the calculated time, which is given for the mean distance of the planet; but when the earth is in the opposite point of the orbit, D, these eclipses happen eight minutes later than the tables predict them. Hence then it was inferred that the motion of light is not instantaneous, but takes about sixteen minutes to pass over a space, equal to the diameter of the earth's orbit, which is about one hundred and ninety millions of miles in length. If, therefore, the sun were to be annihilated, we should see him, as we now see him, eight minutes after that event happened. Hence, it is easy to calculate, how long light is travelling to us from the moon, the other planets, and even from the fixed stars, if their distances could be ascertained. The distances of the latter are,

indeed, immensely great, so that from the nearest of them, suppose Sirius, the dog-star, light must take years even to travel to the earth; and it has been conjectured by some philosophers, that there are stars so remotely situated with respect to the solar system, that the light flowing from them, ever since the creation, and travelling at the rate of 200,000 miles per second, has not even yet reached the earth.

Since the velocity of light is so great, it is justly inferred, that its particles must be almost infinitely small, or the organs of vision would be destroyed by their impulse upon them. The velocity and minuteness of these particles are not more a matter of wonder, than the rarity of the fluid; for its rays cross each other in all possible directions, without the least apparent disturbance. Thus, we can see through a very small pin-hole, in a piece of paper, a variety of objects at the same time. Now the light proceeding from these objects must pass at the same instant through the hole, in a great variety of directions, before they arrive at the eye, yet the vision is not in the least disturbed by it.

Again, if a lighted candle be set, in a dark night, upon an eminence, it may be seen all round to the distance of half a mile; so that there is no place within the sphere of a mile in diameter, in which the eye can be placed, where it will not receive some rays from the flame of this candle.

Another circumstance respecting the rays of light is, that they move always in straight lines, as is evident by the impossibility of seeing through a crooked tube.

As light proceeds from a centre, its intensity decreases as the square of the distance from the luminous body increases; that is, at twice the distance from the luminous body, an object will be enlightened only one-fourth as much as it was before; and at three times the distance, only one-ninth as much, and so on.

By a ray of light is meant the motion of a single particle, and its motion is represented by a straight line.

Any parcel of rays proceeding from a point, is called a

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