bodies falling freely through the operation of gravity; the latter in the case of arrows or rockets propelled vertically upwards.

7. The mechanical powers are the lever, the wheel and the axis, the pulley, the inclined plane, the wedge, and the screw. Of these compounded in different ways the most complete machines are formed.

L. The lever is the simplest of all machines, and is only a straight bar of iron, wood, or other material, supported on and moveable round, a prop_called the fulcrum. In the lever we should consider, 1. The fulcrum, or prop, by which it is supported, or on which it turns as an axis, or centre of motion: 2. the power to raise and support the weight: 3. the resistance or weight to be raised or sustained. There are three kinds of levers. 1. When the fulcrum or prop is placed between the power and the weight as in steel-yards, scissars, pincers, &c. This lever is used to loosen large stones, or to raise geat weights to small elevations. It is the most common species of lever. 2. When the prop is at one end of the lever, the power at the other, and the weight between them. To this kind of lever belong rudders of ships, doors turning upon hinges, &c. The hinges are the centre of motion, the hand applied to the lock is the power, while the door is the weight to be moved. This lever is also applicable to the case of two horses of unequal strength yoked to a carriage; the beam which they pull should be so divided, that the point of traction may be nearer to the stronger horse than to the weaker, in the same proportion as the strength of the for mer exceeds that of the latter. 3. When the prop is at one end, the weight at the other, and the power applied between them. To this sort of lever are generally referred: the bones of a man's arm; for when he lifts a weight by the band, the muscle that exerts its force to raise that weight, is fixed to the boue about one-tenth part as far below the elbow as the haud is. The elbow being the centre round which the lower part of the ann turns, the muscle must therefore exert a force ten times as great as! the weight that is raised. The hammer-lever differs in nos thing but its form, from a lever of the first kind. Its name is derived from its use, that of drawing a nail out of wood

Y

by a hammer.

The method of combining levers is often used in machines, and is of great service.

II. The wheel and axis is a machine much used, and is made in a variety of forms. It consists of a wheel with an axle fixed to it, so as to turn round with it; the power being applied at the circumference of the wheel, and the weight to be raised is fastened to a rope with coils round the axle. Cranes of all kinds, windlasses, and capstans may all be referred to the wheel and axis.

III. The pulley is a small wheel turning on an axis, with a drawing rope passing over it: the small wheel is usually called a sheeve, and is so fixed in a box, or block, as to be inoveable round a pin passing through its centre. Pulleys are of two kinds :-1. Fixed, which do not move out of their places; 2. Moveable, which rise and fall with the weight. A pair of blocks with a rope fastened round it is called a tackle.

IV. The inclined plane. This mechanical power is of very great use in rolling up heavy bodies, such as casks, wheelbarrows, &c. It is formed by placing boards, or earth, in a sloping direction. To the inclined plane belong all hatchets, chisels, and other edge tools.

V. The wedge may be considered as two equally inclined planes, joined together at their bases. The wedge is a very great mechanical power, since not only wood, but even rocks can be split by it; which it would be impossible to effect by the lever, wheel and axle, or pulley. The force of the blow or stroke, shakes the cohering parts aud thus causes them to separate more readily.

VI. The screw, though classed among simple machines, can scarcely be so termed, because it is never used without the application of a lever or winch to assist in turning it. A very considerable degree of friction always acts against the power of a screw; but this is fully compensated by other advantages; for on this account the screw continues to sustain a weight; even after the power is removed, or ceases to act, and presses upon the body against which it is driven. Hence several screws, properly applied, would support a large building, whilst the foundation is inending, or renewed. The screw is of extensive use in the printing press-in the press for coining money-and for a variety of other purposes.

Compound Machines are made by combining together the other mechanical powers, but in these, as well as in simple machines, what is gained in power, is lost in time. If a man, by a fixed pulley, raise a beam to the top of a house in two minutes, it is clear that he will be able to raise six beams in twelve minutes; but by means of a tackle, with three lower pulleys, he will raise the six beams at once, with the same ease as he before raised one; but then he will be six times as long about it, that is, twelve minutes. Thus the work is performed in the same time, whether the mechanical power is used or not. But the convenience gained by the power is very great; for if the six beams are joined in one, they may be raised by the tackle, though it would be impossible to move them by the unassisted strength of one man. No real gain of force is obtained by mechanical contrivances; on the contrary, from friction, and other causes, force is always lost; but by machines we are able to give a more convenient direction to the moving power, and modify its energy.

A pendulum is a body, suspended by a small string, as wire, &c. and swinging backwards and forwards, about some fixed point, by the force of gravity. Each swing of a pendulum is called a vibration or oscillation. The vibrations of a long pendulum are slower than those of a short one, therefore as heat lengthens, and cold contracts the wire on which it swings, a clock will lose time in summer, and gain time in winter. The times of vibrations of pendulums in the same latitude, are as the square roots of their lengths in the latitude of London, the length from the point of suspension to the centre of oscillation, of a pendulum which vibrates seconds is 39 inches.

I.

CHAP. II.-HYDROSTATICS.

THE science of hydrostatics treats of the weight and equilibrium of fluids at rest-when that equilibrium is destroyed, motion ensues; and the science which considers the laws of fluids in motion is hydraulics, or hydrodynamics. Fluids may be divided into two classes, elastic, such as air, vapour, and gas; all which are susceptible of compression proportionally to the force to which they are exposed; and

nelastic, such as water, mercury, spirits, &c. which cannot be compressed; though by being heated they distend considerably. Some philosophers restrain the term fluid to the former of these, and apply the word liquid to the latter. Thus, in their nomenclature, bodies may be found in three different states, i. e. solidity, liquidity, and aëriform fluidity. Fluids press every way alike, though their general tendency is to gravitation. Thus if a vessel be made weaker in the side than at the bottom, and be so laden by the weight of water, as to burst the vessel, the weakest part will become the outlet; but, so soon as it is liberated, the fluid will invariably descend; unless acted upon by a syphon. The pressure upwards is merely in conformity with circumstances attendant upon general pressure, and proves the tendency of fluids to find their own level.

2. The surface of all portions of water, which communicate with one another, will, while in a state of rest, be perfectly level. Water may be carried in pipes to the same height in which it stands in the reservoir; and springs will rise nearly as high as their sources. As fluids press in all directions, their whole weight cannot be applied against one part or side; thus, in a vessel whose base is narrower than its brim, the bottom sustains only the weight of a column equal to its area, multiplied by its height; yet if the pan be of a bell-shape, having its base broader than its brim, the bottom will sustain a weight equal to its area, also multiplied by its height. Consequently in a vessel of a conical form, the base would be oppressed as much as if the sides were cylindrical. This is called the hydrostatic paradox.

3. Specific Gravity. A body is said to be specifically heavier than another, when, under the same bulk, it contains a greater weight than that other; and, reciprocally, the latter is said to be specifically lighter than the former. The specific gravities or densities of bodies are directly as their weights and inversely as their magnitudes. The specific gravity of fluids is ascertained by weighing a known body immersed in them. For the loss by immersion, will accurately show the weight of the same bulk of the fluid. The specific gravity of solids is determined by weighing them first in air, and then in water. The loss of weight,

arising from the action of the water, is equal to that of a mass of the fluid possessing the same dimensions as the solid. The hydrometer, an instrument for obtaining the specific gravity of bodies, is formed of a ball of glass or ivory, to which a graduated stem is attached, and the fluid is heavier or lighter according to the sinking of the ball. The hydrostatic balance is an instrument for comparing the relative weights of solids.

4. The balloon is a hydrostatic, or rather a pneumatic machine, deriving its property of ascending from the air into the upper part of our atmosphere, entirely from the difference between the specific gravity of the air or gas, with which it is filled, and the atmospheric air in which it floats.

1.

CHAP. III.~HYDRAULICS.

THIS science teaches the velocity and force of fluids when in motion, and serves as the basis for computing the powers of various machinery acted on by running water. It comprehends the theory of the motion of water, in pipes, those of spouting fluids, of pumps, of water wheels, of centrifugal machines, of Barker's mill, &c. and since steam engines were first employed to raise water, they are usually classed under this head. Water can only be set in motion by two causes: the increased pressure of the air, and by gravitation: the velocity of water, proceeding through a hole in the side of a vessel, is proportioned to the distance of the aperture from the level of the fluid, the square root of the intermediate space being the guide. If the pressure of the atmosphere be taken away, water will rise above its level, as in the case of the syphon.

2. The syphon is a pipe made of tin or copper, and bent in such a manner that one limb may reach down through the bung-hole of the cask to be emptied, to its very bottom; the other leg should be the longest, so that when filled, it may contain a heavier body of fluid than the limb within the vessel. The pressure of the air being taken from that part of the surface of the liquor within the tube, the liquor rises above its natural level, and flows off through the longer leg, and the contents of the cask are drawn off to the last. Many springs are derived from natural sy