then the two latter, that is, one of the hydrogen and sixteen of sulphur, will combine together.

These laws, it will be perceived, must give great simplicity to chemical combinations. The number of simple substances, which play an important part in such combinations, is small; and, if the proportional numbers for these substances be remembered, as they easily may be, an individual can determine the proportions in which the ingredients combine, in all given compounds, without difficulty, and without having recourse to books.

These laws of affinity, which we have thus endeavored to explain, lie at the foundation of all chemical science, and admit of innumerable applications to the useful arts. They regulate the operations of the bleacher, dyer, tanner, brewer, and baker; assist or mar the labors of the husbandman, glassmaker, metallurgist, manufacturer, &c., and are therefore entitled to the serious attention of all persons connected with these important pursuits. It will be our object, when we come to treat of these arts, to illustrate the applications here referred to, at greater length, and to show, by numerous examples, the necessity of understanding a principle, so perpetually at work, and which is equally powerful, whether employed as an auxiliary or encountered as an antagonist.

CHAPTER II.

MECHANICAL AGENTS EMPLOYED IN THE ARTS.

CHEMICAL agents act only at insensible distances, and are confined, in their operation, to changing the interior constitution of bodies. Mechanical agents, on the contrary, act at sensible distances, leave the interior constitution of bodies unchanged, and alter only the position or form of their masses. These agents are called forces or prime movers; and comprehend the

strength of animals, water, wind, steam, &c. Before we enter upon the examination of these forces, however, it will be necessary to exhibit, in few words, certain fundamental LAWS OF MOTION, which apply to all bodies and forces, alike; and which ought to be thoroughly understood, by every person who is engaged, directly or indirectly, in mechanical operations.

I. The first law is, that masses of matter never change their state of motion or rest, unless external force is applied. This is merely saying, in other words, that matter is inert, or has no power of voluntary action. That a body never passes from a state of rest to that of motion, without the application to it of some force, is evident enough to all. But it is not so evident, that a body, once in motion, would never stop, except from the same cause. This will become apparent, however, if we consider, that a body, rolling over a smooth surface, will continue much longer in motion, than if the surface be rough; and that, if the surface be thoroughly polished, and very hard, a top has been known to continue spinning upon it, for hours. If, in the latter case, we could do away friction, entirely, and remove all the resistance presented by the air, the motion would undoubtedly continue a very long time; and we know of no reason why it should ever stop. It should be added, here, that, as a body once in motion has no power of stopping itself, so neither has it any power of changing its rate of motion; and it will continue, therefore, to move for ever, with uniform velocity, unless some force be applied, to retard or accelerate.

INERTIA OF BODIES.

This law, which is commonly called the law of inertia, suggests some very important rules for regulating motion and machinery, and serves to explain a great many interesting facts. For example: if For example: if you would put in motion a large mass, you must take time; since, each particle of the mass being inert, the inertia of the whole can only be overcome gradually. Hence, a judicious

driver never strikes his horses at starting, lest the sudden exertion of their strength against an inert load should injure them, and break the harness. Hence, also, on railways, they connect the cars by flexible springs, in order that the different cars may be put in motion one after another, instead of compelling the engine or horses to overcome the inertia of the whole train at one effort.

We see the same principle in the case of a large boat lying in water. A sudden pull, though very strong, seems to have no effect upon it; whereas, a slight force, if it be applied steadily, till it has had time to pervade the entire mass, will produce motion. In other cases, however, where our object is not to move the whole mass, but to detach a small part of it, we should apply the force suddenly. Thus, in breaking off fragments from rocks, metals, pottery, &c., the blow must evidently be violent, that the part may be broken off, before the motion communicates to the whole mass. This explains why we can discharge a pistol ball through a pane of glass, without breaking, or even cracking, any part, except just that through which the ball passes. Also, why, if a board be suspended freely, a pistol ball can be driven quite through it, without communicating any sensible motion to the board. Also, why a tallow candle, discharged from a musket, can be driven, like a bullet, through a pine board; and why a cannon ball, coming from a great distance, and moving at a comparatively slow rate, does so much more damage than a shot coming from some point very near.

Thus much, respecting the application of this law of inertia to the case of bodies, which are to be put in motion, and which exhibit a tendency to rest. It admits, also, of many interesting applications to bodies already in motion, and which exhibit a tendency to continue this motion. Thus, it is owing to inertia, that we find it so difficult, when running fast, to stop ourselves, suddenly; that we fall over, forwards, if the moving surface, on which we have been standing, as

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the bottom of a wagon, for example, is unexpectedly stopped; and that we receive such severe falls, when we leap from a carriage in motion to the ground. To this same principle we are to refer the severe concussion communicated even by small bodies, when they are moving rapidly. It should be remarked, here, that the force with which a body moves, and the consequent efforts which it makes to continue in motion, depend on the velocity as well as on the quantity of matter. A hammer is a small body; but, owing to the great velocity which is communicated to it by the arm, as it descends, it is capable of inflicting a very severe blow. So with the flail, in threshing; the balls used in muskets and cannon; the battering ram of the ancients; the pile-engine, &c. Another very interesting application of inertia is in the fly-wheel, which is used in machinery, for the purpose of maintaining a uniform rate of motion, and to which we shall have occasion hereafter to refer.

CENTRIFUGAL FORCE.

The inertia of matter gives rise, also, to a different but very interesting class of facts. If bodies are moving, they have a tendency, as we have seen, to continue in motion, with uniform velocity; and we now add, that this tendency is always to carry them in a right line. Hence, if they move in curves, there is a continual effort to take a rectilinear course, in the direction of tangents to those curves.

This effort is called

the centrifugal force of a body, and has a very important influence on the motion, both of bodies in space and of machinery. We have familiar examples of it, in the water and mud which fly off from the periphery of carriage wheels, when in rapid motion;* in the force with which stones escape from a sling, which has been whirled rapidly round; in the greater liability of a carriage to upset, when it is turning a corner; in the hol

*The velocity given to grindstones, in some manufactories, is so great, that fragments are broken off, by this centrifugal tendency.

low shape which the water in a vessel assumes, if that vessel be revolved quickly round its axis, &c. &c. Valuable use is made of this centrifugal tendency, in the construction of millstones,-the grain always being received between the stones, in the centre, and carried outwards of itself: also, in the lathe which is used by potters and glassmakers; and, above all, in the machine called a governor, which was first applied by Watt, to regulate the supply of steam, or any other moving power, to machinery.

The second law of motion is, that any change in the place of a body must be proportioned to the force impressed, and in the direction of that force. This is sufficiently evident, in the case of a single force, or of two or more forces acting in the same right line, and when the body is free to move in the direction of that line.

COMPOUND MOTION.

There are other cases, however, (as, for instance, that of a boat rowed across a river which has a rapid current,) in which the body is acted upon, at the same time, by two forces at certain angles, either greater than, equal to, or less than, right angles. Thus, if a body at B, Fig. 1, be acted upon, at the same instant, by two forces, at right an- c gles with each other, one impelling it towards C, and the other towards D, it is important to know in what direction it will E

Fig. 1.

B

D

move, and at what rate. Suppose, that the first force, acting alone, would have carried the body through the line B C, in one second, and that the other force, acting alone, would have carried it through B D, in the same time; acting together, they must carry the body, in one second, to a point which shall be just as far from B D as C is, and just as far from B C as D is; that is, to E:* and the body itself will have described the

* For the action of the perpendicular force cannot prevent the full effect of the horizontal; nor, vice versa, would the action of the horThereizontal force prevent the full effect of the perpendicular one.