91 G

THEORY AND PRACTICE OF ENGINEERING.

Book 11.

Motion is a successive and continuous changing of place, and may be accelerated,retarded, or made either absolute or relative.

Repose is the state of matter when in perfect rest.

Space is indefinite, unlimited, and of the extent which all bodies may be supposed to occupy.

Divisibility _All bodies may be brought into the fluid state by heat, but there are

other means by which matter may be divided. Stone, metals, and timber may have theirparticles separated by a blow, by friction, grinding, filing, sawing, splitting, &c. &c. Theseveral divisions so obtained may be in many instances rendered smaller by sublimation ;by passing the finest portions through sieves, and throwing them into water, they may besubjected to decantation; blowing will produce the same effect, the air put in motion bythe bellows carrying the particles to a distance proportionable to their fineness; this is thecase in all powder manufactories, and thus is corn winnowed from the chaff.

Sublimation is vapourising bodies by means of heat in close vessels, and condensing thevapour so formed by cold applications.

Inertia. —As matter in itself is inert or inanimate, it cannot produce motion, or changethat position which is imposed upon it, so that it remains in perfect repose, unless actedupon by an animated mover, or weight or pressure be applied to it: when, however, putin motion, in a certain direction, it will continue to move in it, unless obliged to deviateby some efficient cause ; nor will its rapidity increase unless some acceleration be given.

A weight or heavy body thrown from a height continues to fall with an acceleratedmovement derived from its own weight, which acts continually on it, in the same manneras when it is in repose, and this is rendered evident by the movement of a ball which, whenthrown upwards, diminishes in rapidity instead of augmenting : a cannon ball propelledobliquely changes at each instant of time, the weight operating to bring it to the earth.Inertia obliges a body in constant motion, with a certain rapidity, and in a certain direction,to preserve its line of course, which would be uniform, and in a rectilinear direction, ifnothing occurred to derange it.

Force. — This term is applied to causes which modify the actual state of bodies, orwould do so, if other forces did not interfere to destroy the effect of the first: theresistance of the air, heat, and weight change the state of bodies : different effects offorces are produced; sometimes leaving bodies in a state of repose, sometimes changingtheir form, by breaking or impressing them with movement; at other times acceleratingor retarding that which they possess, or changing its direction ; when bodies suddenlychange their state, either in direction or movement, it arises from an increased force, pro-ducing an effect in a smaller portion of time, the duration of which cannot be measured bythe means we possess. The ball discharged from a gun will pass through a pane of glass,a door panel, a sheet of paper, with so much rapidity, that the parts carried away have nottime to communicate movement to those adjacent.

A cannon attached to a vertical cord, and nicely balanced, sends the ball to itsdestination, in the same manner that it would do if mounted on its carriage; which is aproof that the cannon had not deviated in any appreciable manner before the hall had leftit, although it recoiled afterwards. 'Hie forces which give motion are called moving ; thosethat hasten movement accelerating, and those that keep it back, retarding forces.

Measure, and Nature of Force. —When a body at rest, or free to move is drawn or forcedforward, it may be said to be pressed in that direction ; the effort is called pressure, andanalogous to the force exercised in supporting a weight. Two forces are equal, when,being substituted for each other, and under the same circumstances, they produce the sameeffect, or destroy that of a third v/hon opposed to them. A body suspended by a threadassumes a perpendicular direction ; and if two forces be applied for the same purpose, theywill be equal to each other, as well as to the weight of the body, and may be measured orcompared by means of the balance, or other instrument used for that purpose, amongwhich is the dynometer. The farther we remove a body from the surface of the earth, theless is its apparent weight; so that at the summit of a mountain, the body .suspended to athread will have apparently diminished in weight 1 part in 700 for a height of S miles.The nearer we approach the equator the greater is the diminution of weight, although notto so appreciable a difference as to be of importance in mechanics : the plumb line, or theweight at the end of a thread, is not, strictly speaking, perpendicular, nor are a succession ofthem in parallel lines, as they all tend towards the centre of the earth; but as the distanceis so great, it is unnecessary to take the variation into our consideration. We may estimateweight as a constant force, and invariable in its direction, as far as concerns the labour ofthe mechanic.

The Manner in which Force acts upon a Body. —When force is applied to any point onthe surface of a body, it exerts a pressure against the molecules nearest to this point; andthe body is either compressed, or in some degree gives way. The molecules, when broughtin closer contact, after the force is removed, in virtue of their elasticity, make an effort toreturn to their original state ; by pressing against each other, the force is conveyed to theextreme molecule, ami if this can be dmen no further, then compression takes place, andthe form of the body undergoes a change : should this extreme molecule hav^the power,