Page images




1. MECHANICS is a science which teaches the proportion of the forces, motions, velocities, and, in general, the actions of bodies upon one another.

2. Body is the mass, or quantity of matter, in any material substance; and it is always proportional to its weight or gravity, whatever its figure may


Body is either hard, soft, or elastic. A hard body is such that its parts do not yield to any stroke or percussion, but retain their figure unaltered. A soft body is such that its parts yield to any stroke or impression without restoring themselves again, the figure of the body remaining altered. And an elastic body is such that the parts will yield to any stroke, but which presently restore themselves again, and the body regains the same figure as before the stroke.

There are no bodies that are perfectly hard, soft, or elastic; but all partaking these properties, more or less, in some intermediate degree.


3. Bodies are either solid or fluid. A solid body is such that its parts are not easily moved among one another, and which retains any figure given to it. But a fluid body is such that the parts yield to the slightest impression, being easily moved among one another; and its surface, when left to itself, is always observed to settle in a smooth plane at the top.

4. Density of a body is the proportion of the quantity of matter contained in it, to the quantity of matter contained in another body of the same size or magnitude. Thus, the density is said to be double or triple, when the quantity of matter contained in the same space is double or triple.

5. Force is a power exerted on a body to move it. If it act but for a moment, it is called the force of percussion or impulse. If it act constantly, it is called an accelerative force. If constantly and equally, it is called an uniform accelerative force.

6. Velocity is an affection of motion, by which a body passes over a certain space in a certain time. Thus, if a body in motion pass uniformly over 10 feet in 2 seconds of time, it is said to move with the velocity of 5 feet per second; and so on.

7. Motion is a continual and successive change of place. If a body moves through equal spaces in equal times, it is called equable motion. If its velocity continually increases, it is called accelerated motion. If it decreases, it is retarded motion. If it increases or decreases uniformly, it is equably accelerated or retarded. Likewise if its motion be considered in regard to some other body at rest, it is called absolute motion; but if its motion be considered with respect to other bodies also in motion, then it is relative motion.

8. Direction of motion is the way the body tends, or the right line it moves in.

9. Momentum, or Quantity of Motion, is the power or force in moving bodies by which they continually tend

from their present places, or with which they strike any obstacle that opposes their motion.

10. Forces are distinguished into Motive, or Accelerative or Retardive. A Motive, or moving force, is the power of an agent to produce motion; and it is equal or proportional to the momentum it will generate in a body, when acting either by percussion, or for a certain time as a permanent force.

11. Accelerative or Retardive Force is commonly understood to be that which affects the velocity only, or it is that by which the velocity is accelerated or retarded; and it is equal or proportional to the motive force directly, and to the mass or body moved inversely. So that if a body of 3 pounds weight be acted on by a motive force of 6, then the accelerative force is = 2; but if the same force, 6, act on another body of 6 pounds weight, then the accelerating force in this case is g = 1, and so is but half the former, and will produce only half the velocity.

12. Gravity is that force wherewith a body endeavours to descend towards the centre of the earth. This is called Absolute Gravity when the body tends downwards in free space; and Relative Gravity is the force it endeavours to descend with in a fluid.

13. Specific Gravity is the relation of the weights of different bodies of equal magnitude, and so is proportional to the density of the body.

14. Centre of Gravity of a body is a certain point in it, upon which, the body being freely suspended, it would rest in any position.

15. Centre of Motion of a body is a fixed point about which the body is moved; and the Axis of Motion is the fixed axis it moves about.

16. Weight and Power, when opposed to one another, signify the body to be removed, and the body that moves it. That body which communicates the motion is called the power; and that which receives it, the weight.

17. Statics has for its object the equilibrium of forces applied to solid bodies.

18. By Dynamics* we investigate the circumstances of the motion of solid bodies.

19. Hydrostatics is the science in which the equilibrium of fluids is considered.

20. Hydraulics is the art of raising or conveying water by the help of engines.

21. Pneumatics relates to the properties of elastic fluids, and, according to Def. 19, is a branch of Hydrostatics.


22. First, A body must continue for ever in a state of rest, or in a state of uniform and rectilineal motion, if it be not disturbed by the action of some external cause.

23. Second, The alteration of motion produced in a body by the action of any external force, is always proportional to that force, and in the direction of the right line in which it acts.

24. Third, The action and re-action of bodies on one another are equal, and are exerted in opposite directions. The third law of motion involves two distinct propositions:

1st, When motion is communicated by impulse, the quantity of motion gained by the one body in any direc

*The term Dynamics signifies literally the doctrine of power; power or force being known to us only as the cause of motion, and measured by the motion it produces.

tion, is just equal to that which is lost by the other in the same direction.

2d, When motion is communicated without apparent contact, as in the case of gravitation, and of the phenomena ascribed to attraction or repulsion, the quantity of motion gained by the one body is just equal to that which is gained by the other, but in an opposite direction. Thus, if any two bodies or masses, whatever may be their natures, be placed at rest in free space, at any proposed distance from each other, and beyond the influence of every other body, they will immediately begin to approach each other, but not with a uniform velocity, but with a velocity which is continually accelerated. If the bodies are equal, they will approach each other with equal velocities; but if they are unequal, the velocity of the lesser will be greater than that of the greater in the same proportion as its mass is less; therefore the quantity of motion gained by the one body is just equal to the quantity of motion gained by the other, but in an opposite direction.

Magnetic Attraction is of a similar nature: but while it acts only between some particular bodies, Gravitation acts upon all bodies, of all species or kinds, and under all circumstances; it being a force the intensity of which is totally independent of the nature of the bodies, and only depends on their masses and mutual distances; and, from its universality, it has sometimes been called the Law of Nature, or Universal Gravitation,

« PreviousContinue »