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nically thrown farther forward than when we walk on a horizontal plane, that the line of direction may fall without our feet; and in descending an inclined plane, the body is thrown backward, in order to prevent the line of direction from falling too suddenly without the base. In carrying a burden, the centre of gravity is brought nearer to the burden, so that the line of direction would fall without the feet, if the man did not naturally lean towards the side opposite to the burden, in order to keep the line of direction within the base of the feet. When the burden is carried on the back, the person carrying it leans forward; when it is carried on the right arm, he leans towards the left; when carried on the left arm, he leans towards the right; and when the burden is carried before the body, the head is thrown backwards, for the same purpose.

Friction is the great obstacle to the desired operation of the mechanic powers, in the various machines which human art has invented for the conveniences of social life. The friction generated in the communicating parts of machinery, opposes a great resistance to the impelling power, and is injurious to the machines themselves; and, therefore, many methods have been devised, and are used, for diminishing the friction, and lessening the resistance it opposes to the movements of machines. The most efficacious mode of accomplishing this is, to convert that species of friction which arises from one body being dragged over another, into that which is occasioned by one body rolling upon

another. This may be easily effected by apply. ing wheels, or rollers, to the sockets which sustain the axles of wheels. The effects of friction are diminished, likewise, by a judicious application of the impelling power, and of unguents of grease and tallow, when the surfaces are made of wood; and of oil when they consist of metal. In small works made of wood, the interposition of the powder.of black lead has been found very useful in relieving the friction.

A fly-wheel is a heavy wheel or cylinder, which moves rapidly upon its axis, and is applied to machines, to render uniform a desultory or reciprocating motion, arising either from the nature of the machinery, or from an inequality in the resistance to be overcome, or from an irregular application of the impelling power. When the moving power is inanimate, as wind, water, or steam, an inequality of force may arise from variations in the velocity of the wind, from the increase or decrease of water, or from the augmentation or diminution of steam. The same

inequality of force may take place, alsó, when machines are moved by animals. A fly-wheel, consisting either of cross bars, or a massy, circular rim, removes this inconvenience.

QUESTIONS.

What is the centre of gravity? What is the line of direction? How must the line of direction be situated to prevent any body from falling? What situation of the centre of gravity renders a body most liable to be overset? When are bodies said to have a stable equilibrium? When are bodies said to have a tottering equilibrium? When are bodies said to have a neutral equilibrium? How do bodies stand most firmly?

What phenomena in the equilibrium and motions of animals are deducible from the properties of the centre of gravity? What is the great obstacle to the free operation of the mechanic powers? By what methods is friction diminished? What is the fly-wheel?

CHAP. XXXVII.

HYDRODYNAMICS.

HYDRODYNAMICS, a term compounded of two Greek words, signifying water and power, is given to that science which treats of the power of water, whether acting by pressure or by impulse. In its more enlarged acceptation, however, it treats of the pressure, equilibrium, cohesion, and motion of fluids, and of the machines by which water is raised, or in which it is employed as the first mover. Hydronamics is divided into two branches; hydrostatics and hydraulics. Hydrostatics contemplates the pressure, equilibrium, and cohesion of fluids; and hydraulics, their motions, with the machines in which they are chiefly concerned.

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Hydrostatics regards the pressure and equilibrium of non-elastic fluids, as water, oil, mercury, and others such; the method of determining the specific gravities of substances, the equili brium of floating bodies, and the phenomena of capillary attraction.

A fluid is a collection of very minute particles, adhering so little among themselves as to yield to

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the smallest force, and to be easily moved among one another.

Fluids have been divided into perfect and imperfect. In perfect fluids, the constituent particles are supposed to be endowed with no cohesive force, and to be moved among one another, by a pressure infinitely small. In imperfect, or viscous fluids, the mutual cohesion of their particles is very sensible, as in oil, varnish, melted glass, &c. and this tenacity prevents them from yielding to very small pressure.

Till within some few years, it was generally believed that water, mercury, and other fluids of a similar kind, could not be made to occupy a smaller space, by the application of any external force. This opinion was founded on an experiment made by Bacon, Lord Verulam, who inclosed a quantity of water in a leaden globe, and by applying a great force, endeavoured to compress the water into less space than it occupied at first. The water made its way through the pores of the metal, and stood upon its surface like dew. A similar experiment was performed at Florence. A silver globe was filled with water, and by means of the screw, a violent power was made to act upon it. As in Bacon's experiment, the water was driven through the pores of the metal. In consequence of the reliance placed on these experiments, fluids have been divided into compressible and incompressible; or elastic and non-elastic.

Water, oil, alcohol, and mercury, were regarded as incompressible and non-elastic; and air, steam, and other aëriform fluids, as com

pressible or elastic. But subsequent to this, experiments made by Canton and Zimmerman, have proved that fluids are capable of contraction and dilation, and that there is no foundation in nature for their being divided into compressible and incompressible. If fluids are compressible, they must also be elastic; for when the compressing force is removed, they will recover their former magnitude. Hence their division into elastic and non-elastic is equally improper.

When fluids are subjected to any pressure, that pressure is diffused throughout the mass in such a manner, that when it remains in equilibrio all its parts are equally pressed in every direction.

The pressure of fluids acts equally in every direction, upwards, sideways, and downwards.

The surface of a fluid influenced by the force of gravity, and in equilibrio, in any vessel, is horizontal, or at right angles to the direction of gravity.

The surface of a fluid influenced by the force of gravity, and contained in any number of vessels communicating with one another, however different they may be in form and position, will be horizontal.

This principle is the cause that fluids always find their level, when not obstructed in their

natural pressure. Upon this property depends

the natural rise of water in fountains and tubes, to the height of the spot and spring whence it first issues. This explains, also, the reason why the surface of small pools in the vicinity of

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