tation, and of central forces, as they appear to operate in the motions of the celestial bodies; and on the surface of our globe, in the phenomena of falling bodies, the motions of projectiles, the vibration of pendulums, &c.-the theory of machines, the principles on which their energy depends; the properties of the mechanical powers-the lever, the wheel and axle, the pulley, the inclined plane, the wedge and the screw-and the effects resulting from their various combinations. From the investigations of philosophers on these subjects, we learn the laws by which the great bodies of the universe are directed in their motions; the laws which bind together the different portions of matter on the surface of the earth, and which regulate the motions of animal, vegetable, and inanimate nature; and the principles on which cranes, mills, wheel-carriages, pile engines, thrashing-machines, and other engines, are constructed; by means of which, man has been enabled to accomplish operations far beyond the limits of his own physical powers.

Without a knowledge of the laws of motion, and assistance from the combined effects of the mechanical powers, man would be a very limited being, his enjoyments would be few, and his active energies confined within a very narrow range. In a savage state, ignorant of manufactures, agriculture, architecture, navigation, and the other arts which depend upon mechanical combinations, he is exposed, without shelter, to the inclemencies of the seasons; he is unable to transport himself beyond seas and oceans, to visit other climes, and other tribes of his fellow-men; he exists in the desert, comfortless and unimproved; the fertile soil over which he roams, is covered with thorns, and briars, and thickets, for the haunt of beasts of prey; his enjoyments are little superior to those of the lion, the hyæna, and the elephant, while he is much their inferior in point of agility and physical strength. But, when Philosophy has once demonstrated the principles of Mechanics, and introduced the practice of the useful Arts, "the wilderness and the solitary place are made glad, and the desert rejoices, and blossoms as the rose." Cities are founded, and gradually rise to opulence and splen

dour; palaces and temples are erected; the damp cavern, and the rush-built hut, are exchanged for the warm and comfortable apartments of a substantial mansion; ships are built, and navigated across the ocean; the treasures of one country are conveyed to another; an intercourse is carried on between the most distant tribes of mankind; commerce flourishes, and machinery of all kinds is erected, for facilitating human labour, and promoting the enjoyments of man. And, when the principles and the practice of "pure and undefiled religion" accompany these physical and mechanical operations, love and affection diffuse their benign influence; the prospect brightens as years roll on, and man advances, with pleasure and improvement, to the scene of his high destination.

II. HYDROSTATICS treats of the pressure and equilibrium of fluids. From the experiments which have been made in this branch of philosophy, the following important principles, among many others, have been deduced :

(1.) That the surface of all waters which have a communication whilst they are at rest, will be perfectly level.—This principle will be more clearly understood by an inspection of the following figures. If water be poured into the tube A, (Fig. 1.) it will run through the horizontal tube E, and rise in the opposite tube B, to the same height at

which it stands at A. It is on this principle that water is now conveyed underground, through conduit pipes, and made to rise to the level of the fountain whence it is drawn. The city of Edinburgh, a considerable part of which is elevated above the level of the surrounding country, is supplied with water from a reservoir on the Pentland hills, several miles distant. The water is conveyed in leaden pipes down the declivity of the hill, along the intervening plain, and up to the entrance of the castle, whence it is distributed to all parts of the city. If the point A, represent the level of the reservoir, C D, will represent the plain along which the water is conveyed, and B, the elevation to which it rises on the castle hill. On the same principle, and in a similar manner, the city of London is supplied with water from the water-works at London Bridge. Had the ancients been acquainted with this simple, but important principle, it would have saved them the labour and expense of rearing those stupendous works of art, the Aqueducts, which consisted of numerous arches of a vast size, and sometimes piled one above another.

Fig. 2. represents the syphon, the action of which depends upon the pressure of the atmosphere. If this instrument be filled with water, or any other liquid, and the shorter leg G, plunged to the bottom of a cask, or other vessel containing the same liquid, the water will run out at the longer leg F, till the vessel be emptied, in consequence of the atmospheric pressure upon the surface of the liquid. On this principle, water may be conveyed over a rising ground to any distance, provided the perpendicular height of the syphon above the surface of the water in the fountain, does not exceed 32 or 33 feet. On the same principle are constructed, the fountain at command, the cup of Tantalus, and other entertaining devices. The same principle too enables us to account for springs which are sometimes found on the tops of mountains, and for the phenomena of intermitting springs, or those which flow and stop by regular alternations.

(2.) Any quantity of fluid, however small, may be made to counterpoise any quantity, however large. This is what has generally been termed the Hydrostatical Paradox; and

from this principle it follows, that a given quantity of water may exert a force several hundred times greater or less, according to the manner in which it is employed. This force depends on the height of the column of water, independent of its quantity; for its pressure depends on its perpendicular height. By means of water conveyed through a very small perpendicular tube, of great length, a very strong hogshead has been burst to pieces, and the water scattered about with incredible force. On this principle, the hydrostatic press, and other engines of immense power, have been constructed.

(3.) Every body which is heavier than water, or which sinks in it, displaces so much of the water as is equal to the bulk of the body immersed in the water. On this principle, the specific gravities, or comparative weight, of all bodies are determined. It appears to have been first ascertained by Archimedes, and by means of it he determined that the golden crown of the king of Syracuse had been adulterated by the workmen. From this principle we learn, among many other things, the specific gravity of the human body; and that four pounds of cork will preserve a person weighing 135 pounds from sinking, so that he may remain with his head completely above water.

Hydraulics, which has sometimes been treated as a distinct department of mechanical philosophy, may be considered as a branch of Hydrostatics. It teaches us what relates to the motion of fluids, and how to estimate their velocity and force. On the principles of this science, all machines worked by water are constructed as steam engines, water-mills, common and forcing pumps, syphons, fountains, and fire-engines.

III. PNEUMATICS.-This branch of philosophy treats of the nature and properties of the atmosphere, and their effects on solid and fluid bodies. From this science we learn, that air has weight, and presses on all sides, like other fluids; that the pressure of the atmosphere upon the top of a mountain, is less than in the plain beneath; that it presses upon our bodies with a weight of several thousand pounds more at one time than at another; that air can be compressed into forty thousand times less space

than it naturally occupies; that it is of an elastic or expansive nature, and that the force of its spring is equal to its weight; that its elasticity is increased by heat; that it is necessary to the production of sound, the support of flame and animal life, and the germination and growth of all kinds of vegetables.

These positions are proved and illustrated by such experiments as the following:-The general pressure of the atmosphere is proved by such experiments as those detailed in No. II. of the Appendix. The following experiment proves that air is compressible. If a glass tube, open at one end, and closed at the other, be plunged, with the open end downwards, into a tumbler of water, the water will rise a little way in the tube; which shows, that the air which filled the tube is compressed by the water into a small space. The elasticity of air is proved by tying up a bladder, with a very small quantity of air within it, and putting it under the receiver of an air-pump when it will be seen gradually to inflate, till it becomes of its full size. A similar effect would take place, by carrying the bladder to the higher regions of the atmosphere. On the compression and elasticity of the air, depends the construction of that dangerous and destructive instrument, the Air-gun. That it is capable of being rarified by heat, is proved by holding to the fire a half-blown bladder, lightly tied at the neck, when it will dilate to nearly its full size; and if either a full-blown bladder, or a thin glass bubble filled with air, is held to a strong fire, it will burst. The elasticity of the air is such, that Mr. Boyle, by means of an air-pump, caused it to dilate till it occupied fourteen thousand times the space that it usually does.That air is necessary to sound, flame, animal and vegetable life, is proved by the following experiments: When the receiver of an air-pump is exhausted of its air, a cat, a mouse, or a bird, placed in it, expires in a few moments, in the greatest agonies. A bell rung in the same situation, produces no sound; and a lighted candle is instantly extinguished. Similar experiments prove that air is necessary for the flight of birds, the ascent of smoke and vapours, the explosion of gun-powder, and the growth of