GENERAL PROPERTIES OF MATter, &c.

41

should keep pace with the sun, it is evident he would have continual day, or it would be the same day to him during his tour round the earth; but the people who remained at the place he departed from, have had night in the same time, consequently, they reckon a day more than he does.

24. Hence, if two ships should set out at the same time from any port, and sail round the globe, the one eastward and the other westward, so as to meet at the same port on any day whatever, they will differ two days in reckoning their time at their return. If they sail twice round the earth they will differ four days; if thrice, six, &c.

25. But, if two ships should set out at the same time from any port and sail round the globe, northward or southward, so as to meet at the same port on any day whatever, they will not differ a minute in reckoning their time, nor from those who reside at the port.

+ CHAPTER II.

f the General Properties of Matter and the Laws of

Motion.

1. MATTER is a substance which, by its different. modifications, becomes the object of our five senses; viz. whatever we can see, hear, feel, taste, or smell, must be considered as matter, being the constituent parts of the universe.

2. THE PROPERTIES OF MATTER are extension, figure, solidity, motion, divisibility, gravity, and vis inertiæ. These properties, which Sir Isaac Newton observes* are the foundation of all philosophy, extend to the minutest particles of matter.

3. EXTENSION, when considered as a property of matter, has length, breadth, and thickness.

4 FIGURE is the boundary of extension; for every finite extension is terminated by, or comprehended under, some figure.

*Newton's Princip. Book III-The third rule of reasoning in philosophy.

5. SOLIDITY is that property of matter by which it fills space; or, by which any portion of matter excludes every other portion from that space which it occupies. This is sometimes defined the impenetrability of mat

ter.

6. MOTION. Though matter of itself has no ability to move; yet, as all bodies, upon which we can make suitable experiments, have a capacity of being transferred from one place to another, we infer that motion is a quality belonging to all matter.

7. DIVISIBILITY of matter signifies a capacity of being separated into parts, either actually or mentally. That matter is thus divisible, we are convinced by daily experience, but how far the division can be actually carried on is not easily seen. The parts of a body may be

so far divided as not to be sensible to the sight; and by the help of microscopes we discover myriads of organized bodies totally unknown before such instruments were invented. A grain of leaf gold will cover fifty square inches of surface,* and contains two millions of visible parts; but the gold which covers the silver wire, used in making gold lace, is spread over a surface twelve times as great. From such considerations as these, we are led to conclude, that the division of matter is carried on to a degree of minuteness far exceeding the bounds of our faculties.

Mathematicians have shewn that a line finitely divided, as follows:

Draw any line A C, and another A BM perpendicular to it, of an unlimited length towards Q; and from any point D, in A C, draw D E parallel to B M. Take any number of points, P, O, N, M, in BQ; then from P as a centre, and the distance P B, describe the arch Bp, and in the same manner with O, N, M, as centres, and distan

may

be inde

B

DC

m n

ces OB, NB, and M B, describe the arches Bo, Bn, Bm. Now it is evident the farther the centre is taken from B, the nearer the arches will approach to D, and the line E D will be divided into parts, each smaller

* Adams' Natural and Experimental Philosophy. Lect. XXIV.

than the preceding one; and since the line B M may be extended to an indefinite distance beyond Q, the line ED may be indefinitely diminished, yet it can never be reduced to nothing, because, an arch of a circle can never coincide with the straight line BC, hence it follows that ED may be diminished ad infinitum.

8 GRAVITY is that force by which a body endeavours to descend towards the centre of the earth. By this power of attraction in the earth, all bodies on every part of its surface are prevented from leaving it altogether, and people move round it in all directions, without any danger of falling from it. By the influence of attraction, bodies, or the constituent parts of bodies accede, or have a tendency to accede to each other, without any sensible material impulse, and this principle is universally disseminated through the universe, extending to every particle of matter.

9. VIS INERTIA is that innate force of matter by which it resists any change. We cannot move the least particle of matter without some exertion, and if one portion of matter be added to another the inertia of the whole is increased: also, if any part be removed the inertia is diminished. Hence, the vis inertiæ of any body is proportional to its weight.

10. ABSOLUTE AND RELATIVE MOTION. A body is said to be in absolute motion, when its situation is changed with respect to some other body, or bodies, at rest; and to be relatively in motion, when compared with other bodies which are likewise in motion.

When a body always passes over equal parts of space in equal successive portions of time, its motion is said to be uniform.

When the successive portions of space, described in equal times, continually increase, the motion is said to be accelerated; and if the successive portions of space continually decrease, the motion is said to be retarded. Also, the motion is said to be uniformly accelerated or retarded, when the increments or decrements of the spaces, described in equal successive portions of time, are alwavs equal.

11. THE VELOCITY of a body, or the rate of its motion, is measured by the space uniformly described in a given time.

12. FORCE. Whatever changes, or tends to change, the state of rest or motion of a body is called force. If a force 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.

GENERAL LAWS OF MOTION.

LAW I." Every body perseveres in its state of rest, "or uniform motion in a right line, unless it is compel"led to change that state by forces impressed thereon." -Newton's Princip. Book I.

A

B

Thus, when a body A is positively at rest, if no external force put it in motion it will always continue at rest. But if any impulse be given to it in the direction A B, unless some obstacle, or new force, stop or retard its motion, it will continue to move on uniformly, for ever, in the same direction A B. Hence any projectile, as a ball shot from a cannon, an arrow from a bow, a stone cast from a sling, &c. would not deviate from its first direction, or tend to the earth, but would go off from it in a straight line, with an uniform motion, if the action of gravity and the resistance of the air did not alter and retard its motion.

LAW II. The alteration of motion, or the motion "generated or destroyed, in any body, is proportional "to the force applied; and is made in the direction of "that straight line in which the force acts."-Newton's Princip. Book I.

Thus, if any motion be generated by a given force, a double motion will be produced by a double force, a triple motion by a triple force, &c. and considering motion as an effect, it will always be found that a body receives its motion in the same direction with the cause that acts upon it.-If the causes of motion be various, and in different directions, the body acted upon must take an oblique or compound direction. Hence a curvilinear motion cannot be produced by a simple cause, but must arise from the joint effect of different causes, acting at the same instant upon the body.

LAW III." To every action there is always opposed "an equal re-action; or the mutual actions of two "bodies upon each other are always equal, and directed "to contrary points."-Newton's Princip Book I..

If we endeavour to raise a weight by means of a lever, we shall find the lever press the hands with the same force which we exert upon it to raise the weight. Or if we press one scale of a balance, in order to raise a weight in the other scale, the pressure against the finger will be equal to that force with which the other scale endeavours to descend.

When a cannon is fired, the impelling force of the powder acts equally on the breech of the gun and on the ball, so that if the piece and the ball were of equal weight, the piece would recoil with the same velocity as that with which the ball issues out of it. But the heavier any body is the less will its velocity be, provided the force which communicates the motion continues the same. Therefore so many times as the cannon and carriage are heavier than the ball, just so many times will the velocity of the cannon be less than that of the ball,

COMPOUND MOTION.

1. If two forces act at the same time on any body, and in the same direction, the body will move quicker than it would by being acted upon by only one of the forces,

2. If a body be acted upon by two equal forces, in exactly opposite directions, it will not be moved from its

situation.

3. If a body be acted upon by two unequal forces, in exactly contrary directions, it will move in the direction of the greater force.

4. If a body be acted upon by two forces, neither in the same nor opposite directions, it will not follow either of the forces, but move in a line between them.

The first three of the preceding articles may be considered as axioms, being self evident; the fourth may be thus elucidated: Let a force be applied to a body at