Chap. XI.

MECHANIC'S*

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however, to move, it will advance, and the movement will be communicated to the parts ofthe whole body, and these approaching nearer and nearer, in consequence of a series of com-pressions, indicate clearly that time is necessary before any force can communicate its totaleffect ; for rapidity of movement is not instantaneous, as has been too often supposed. Thesame happens when an inverse force is employed to destroy the movement of a body ; themolecules nearest the point of action are first stopped, and then the others in succession.

As each molecule is to a certain degree elastic, it cannot be compressed without a reactiontaking place, for the force which presses against a l>ody is repulsed in the same manner asit is exerted in forcing that body forward ; this reaction is equal, though contrary to,the action itself.

When a body is pressed by the finger, or drawn by a thread, or pushed with a rod, wehave by the same effort pressed, drawn or pushed the body in contrary directions. In allcases, the action of the force is transmitted to the point of resistance by a series of reactions,equal and opposite, destroying each other : but we admit generally that the force appliedto a given point exerts itself over the whole.

Inertia of Matter , measured by means of the Forces -W hen an external force acts on a

body free to move, or destroys the motion imposed upon it, the resistance offered is equalto the force applied ; by this resistance we measure the inertia of matter ; in the samebody the resistance is increased, according to the degree of rapidity impressed or destroyed,and this quality is augmented in proportion to the matter contained in each body. If aweight be drawn by a cord, the cord extends, elongates, and perhaps snaps, when suddenlypulled ; if it be suspended, and in the line of suspension a spring balance is introduced, thespring will indicate the weight of the body ; this weight taken away, the spring will againresume its state when in repose ; the variations of the movement of a body and the force ofinertia are thus measured. This measurement may be expressed by making lines equal inlength to the force, in pounds or any other weight; or a number of divisions marked upona given line may show the intensity of the force; and thus mechanics may be studied byfigures in geometry.

Equilibrium of Forces _When two or more forces are applied, they reciprocally destroy

each other, so that they leave the body to which they are applied in the same state as pre-vious to their application, whether in action or repose ; and if they neither modify thedirection nor the intensity of the rapidity when the body is in motion, then there is anequilibrium between the forces. This equilibrium is static in the first instance, anddynamic in the second ; if a cord be drawn or stretched with equal force at each end, it isin a state of equilibrium ; but if a greater force be applied at one end, then it is dynamic ;there is no true static equilibrium, the earth attracting all bodies to a certain degree-

Influence of Inertia on the state of Equilibrium. —hen a body is acted on by several externalforces, and preserves a uniform movement, it is considered in equilibrio. When therapidity is augmented or diminished, the equilibrium has no place in these forces ; but ifregard be had to the force of inertia of the different molecules, and we substitute anexterior force equal to those originally applied, there will then be an equilibrium betweenall these forces : a horse drawing a carriage destroys at every instant the resistance offeredto his action, and if his movement is uniform, all the obstacles presented by the inequalityof the road are overcome : if his rapidity he increased, the inertia put in motion is addedto the preceding resistance, and the horse causes an equilibrium to take place among allthese forces : if, however, the rapidity he diminished, the inertia which keeps the carriagein motion adds its action to that of the horse, and destroys or maintains the resistances inequilibrio.

When two forces are directly opposed, the less will destroy the greater to an amountequal to its value. Three men pulling a cord in the same direction, with the force of 20,34, and 50 pounds, in all 104 pounds; and two others pulling in an opposite direction,with a force of 24 and 38 pounds, total 62 pounds ; the cord will really be drawn with aforce of 104 — 62 or 42 pounds, acting in the direction of the three men.

Mechanical Labour of the Forces -The most simple state of equilibrium is when two

equal forces destroy each other, and this is the case in works of industry- To work is toovercome or destroy resistance, such as the force of adhesion, which exists between the mole-cules of bodies, the force of springs, of weight, the inertia of matter, See., Sec. : to polish abody by friction, to divide it into parts, to raise weights, draw a carriage, bend a spring,propel a ball, are all works in which labour is constantly renewed, to overcome the re-sistance offered : in mechanical labour, resistance is overcome at the same time that it isreproduced. To remove a portion of matter from a mass by means of any tool, it isrequisite not only to produce an effect directly opposed to the resistance presented, butthe tool must he made to advance in the direction of the resistance ; the greater this ad-vancement is, the greater will be the quantity of matter removed ; at the same time it mustbe taken into account that the greater the length or thickness acted upon, the greater willbe the resistance, so that the work increases with the intensity of the effort and the lengthof the direction.

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