continually divided. That is, a portion of matter may be divided, and each part again divided, and each of the parts divided again, and so on, continually, without ever arriving at a portion which will be absolutely nothing.

Suppose, for instance, you take a portion of matter, say one pound or one ounce, and divide it into two equal parts, and then divide each part again into two equal parts, and so on continually. Now, all the parts will continually grow smaller and smaller, but no one of them will ever become equal to nothing, since the half of a thing must always have some value.

11. What is inertia?

INERTIA is the resistance which matter makes to a change of state. Bodies are not only incapable of changing their actual state, whether it be that of motion or rest, but they seem endowed with the power of resisting such a change. This property is called inertia.

12. If a body is at rest, will it remain so? If in motion, will it continue so?

If a body is at rest it will remain so, unless something be applied from without to move it; and if it be moving, it will continue to move, unless something stops it.

13. What are atoms?

The smallest parts into which we can suppose a body divided, are called particles or atoms.

14. Do these atoms adhere to each other? They do, and form masses or bodies.

15. What is the force called which unites them?

It is called the attraction of cohesion. Without this power solid bodies would crumble to pieces and fall to atoms.

16. In what kind of bodies does this attraction exist?

In all bodies, fluid as well as solid. It is the attraction of cohesion which holds a drop of water in suspension at the end of the finger, and causes it to take a spherical form.

The attraction of cohesion is stronger in some substances than in others. Those in which it is the weakest are easily broken, or the attraction is easily overcome; while those in which it is greater, are proportionably stronger.

17. What is the difference between the attraction of cohesion and the attraction of gravitation?

The attraction of cohesion unites the particles of matter, and these by their aggregation form masses or bodies. The attraction of gravitation is the force by which masses of matter tend to come together. The attraction of cohesion acts only between particles of matter which are very near each other, while the attraction of gravitation acts between bodies widely separated.

18. Is the attraction between bodies mutual?

The attraction between two bodies is mutual; that is, each body attracts the other just as much, and no more, than it is attracted by it. But if the bodies are left free, the smaller will move towards the larger; for, as they are urged together by equal forces, the smaller will obey the force faster than the larger. Thus, the earth being larger than any body near its surface, forces all bodies towards it, and they immediately fall unless the attraction of gravitation is counteracted.

It should, however, be borne in mind that every body attracts the earth just as strongly as the earth attracts the body; and the body moves towards the earth, only because the earth is larger, and therefore not as rapidly moved by their mutual attraction.

19. What is weight?

Weight is the force which is necessary to overcome the attraction of gravitation. Thus, if we have two bodies, and one has twice as much tendency to descend towards the earth as the other, it will require just twice as much force to support it, and hence we say that it is twice as heavy.

SECTION II.

LAWS OF MOTION, AND CENTRE OF GRAVITY.

1. What is motion?

Motion is a change of place. Thus, a body is said to be in motion when it is continually changing its place.

2. Can a body put in motion stop itself?

It has been observed in Art. 11, that bodies are indifferent to rest or motion. Hence, a body cannot put itself in motion, or stop itself after it has begun to move.

3. What is force or power?

That which puts a body in motion, or which changes its motion after it has begun to move, is called force or power. Thus, the stroke of the hammer is the force which drives the nail, the effort of the horse the force which moves the carriage, and the attraction of gravitation the force which draws bodies to the earth.

4. What is velocity?

The rate at which a body moves, or the rapidity of its motion, is estimated by the space which it passes over in

a given portion of time, and this rate is called its velocity. Thus, if in one minute of time a body passes over 200 feet, its velocity is said to be 200 feet per minute; and if another body, in the same time, passes over 400 feet, its velocity is said to be 400 feet per minute, or double that of the first.

5. What is uniform velocity?

When a body moves over equal distances in equal times, its velocity is said to be uniform. Thus, if a body move at the rate of 30 feet a second, it has a uniform velocity, for it always passes over an equal space in an equal time.

6. What is uniformly-accelerated velocity?

Bodies which receive uniform accelerations of velocity, that is, equal accelerations in equal times, are said to have motions uniformly accelerated.

7. How will a body fall by the attraction of gravitation?

If a body fall freely towards the earth, by the attraction of gravitation, it will descend in a line perpendicular to its surface.

8. How far will it fall in the first second, and how far in each succeeding second? What kind of velocity will it have?

In the first second it will fall through 16 feet; in the second second, having the velocity already acquired, and being still acted on by the force of gravity, it will descend through 32 feet; in the third second it will descend through 48 feet; in the fourth second through 64 feet, and so on, adding to its velocity in every additional second. This is a motion uniformly accelerated, for the velocity is equally increased in each second of time.

9. What is momentum?

Momentum is the force with which a body in motion would strike against another body. If a body of a given weight, say 10 pounds, were moving at the rate of 30 feet per second, and another body of the same weight were to move twice as fast, the last would have double the momentum of the first.

10. On what does the momentum of a body depend?

When the bodies are of a given weight, the momentum will depend on the velocity. But if two unequal bodies move with the same velocity, their momentum will depend upon their weight. Hence, the momentum of a body will depend on its weight and velocity; that is, it will be equal to the weight multiplied by the velocity.

If the weight of a body be represented by 5, and its velocity by 6, its momentum will be 5 × 6 = 30.

If the weight of a body be represented by 8, and its velocity by 2, its momentum will be represented by

16 x 2 = 32.

11. What are action and reaction, and how do they compare with each other?

When a body in motion strikes against another body, it meets with resistance. The force of the moving body is called action, and the resistance offered by the body struck is called reaction; and it is a general principle, that action and reaction are equal. Thus, if you strike a nail with a hammer, the action of the hammer against the nail is just equal to the reaction of the nail against the ham mer. Also, if a body fall to the earth, by the attraction of gravitation, the action of the body when it strikes the earth is just equal to the reaction of the earth against the body.