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little oil between them to fill up the pores in their surfaces, they will cohere so powerfully as to require a very considerable force to separate them. Two globules of quicksilver, placed very near to each other, will run together, and drops of water will do the same. The ascent of water and other liquids in sugar, sponge, and all porous bodies is a species of this attraction, and is called capillary attraction.

Some bodies appear to possess a power which is the reverse of the attraction of cohesion. It is called repulsion, and is supposed to extend to a small distance around bodies, so as to prevent them from coming into actual contact. Water repels most bodies till they are wet. A small needle carefully placed on water will float. The drops of dew which appear in the morning on plants assume a globular form, from the mutual attraction between the particles of water; and upon examination it will be found that the drops do not touch the leaves, for they roll off in compact bodies, which would not be the case if there existed any degree of attraction between the water and the leaf. The repelling force between water and oil is so great that it is impossible to mix them in such a manner that they shall not separate again.

QUESTIONS.-1. What is matter? 2. What are the general properties of bodies? 3. What is impenetrability? 4. By what experiments is this property of matter illustrated? 5. Define extension and figure. 6. What is divisibility, and how illustrated? 7. Define inertness? 8. What is meant by attraction? 9. Attraction of cohesion ? 10. What is said of the attraction of solids and fluids? 11. What experiments illustrate cohesive attraction? 12. What is capillary attraction? 13. What is repulsion, and by what experiments illustrated.

LESSON 16.

Attraction of Gravitation.

Rectilinear, consisting of right or straight lines.
Curvilinear, consisting of crooked, or curved lines.
Projec tile, a body put in motion.

Evaga/tion, a wandering deviation.

Phenom'enon, (pl. phenomena) appearance, commonly expressive of some remarkable appearance in nature.

The attraction of gravitation is only a modification of the

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attraction of cohesion. The latter is not perceptible but in very minute particles, and at very small distances, the other acts on the largest bodies, and extends to immense distances.

That very law which moulds a tear,
And bids it trickle from its source,
That law preserves the earth a sphere,

And guides the planets in their course.-ROGERS.

The tendency which bodies have to fall is produced entirely by the attraction of the earth; for the earth is so much larger than any body, on its surface, that it forces every body, which is not supported, to fall upon it. The following simple incident led to the most extensive and complicated calculations, and was productive of the most noble and wonderful discoveries. Newton happening one day, in the year 1666, when only twenty-five years of age, to be sitting under an apple-tree, and an apple falling upon his head, it suggested a variety of reflections. The phenomena of falling bodies. in particular engaged his attention; and, extending his researches to the heavens, he began to investigate the nature of motion in general. Because there is motion, he reasoned, there must be a force that produces it. But what is this force? That a body when left to itself, will fall to the ground, is known to the most ignorant; but if you ask them the reason of its thus falling, they will think you either an idiot or a madman. The circumstance is too common to excite their wonder, although it is so embarrassing to philosophers, that they think it almost inexplicable. It is the mark of a superior genius to find matter for wonder, observation, and research, in circumstances which to the ordinary mind appear trivial, because they are common, and with which they are satisfied, because they are natural, without reflecting that nature is our grand field of observation, that within it is contained our whole store of knowledge; in a word, that to study the works of nature, is to learn to appreciate and admire the wisdom of God.

In applying his reflections on the nature of falling bodies to the celestial motions, Newton soon perceived that the force of gravity was not confined to the surface of our globe; it being found to act alike at the bottom of the lowest valleys, and at the summit of the most lofty mountains. This led

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him to conjecture, that it might extend as far as the moon, and be the means of retaining her in her orbit. Imagine the moon, he reasoned, at the first moment of its creation, to have been projected forward, with a certain velocity, in a rectilinear direction; then, as soon as it began to move, gravity would act upon it, and impel it toward the centre of the earth. But as a body, impelled by two forces, will follow the direction of neither, the moon, so circumstanced, would neither proceed directly forward, nor fall directly downward, but keep a middle course, and move round the earth in a curvilinear orbit. This may be more fully illustrated, by attending to the motion of a shot, or any other projectile. A ball, shot from the mouth of a cannon, in a horizontal direc tion, does not fall to the ground till it has proceeded to a considerable distance; and if it be discharged from the top of a high mountain, it will fly still further before it comes to the earth. Increase the force and the height, and the dis tance will be augmented accordingly. And thus, in imagination at least, we can suppose the ball to be discharged with such velocity, that it will never come to the ground, but return to the place whence it set out, and circulate continually round the earth, in the manner of a little moon. Thus proceeding in his reflections, Newton discovered the admirable provision of the great Creator to prevent the evagation of the planets, and to retain them exactly within the bounds of their orbits. This he has demonstrated to be effected by gravity, and that gravity and motion completely solve all the phenomena of the planetary revolutions, both primary and secondary. By establishing this one principle in philosophy he has fully explained the system of the world, so far as it relates to this globe, and to all the rest of the planets that regard the sun as their centre. Such is the Newtonian system of universal gravitation or attraction. But what is this principle, which gives life and motion to inanimate beings, and how does it act? The effects are visible, but the agent that produces them is hidden from our senses. It eluded the search of Newton himself; he that soared to the utmost regions of space, and looked through nature with the eye of an eagle, was unable to discover it. This principle of gravitation, has been styled "The constant impression of Divine power;"-in every other sense the cause is likely to continue unexplored by man. It is, however, pretty ge

CENTRE OF GRAVITY.

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nerally agreed that the same principle of gravity, by which we see all bodies tend toward the centre of the earth, is a general law of nature, extended to all distances, and to every body, or substance, in the universe.

For this the moon thro' heaven's blue concave glides,
And into motion charms th' expanding tides,
While earth impetuous round her axle rolls,

Exalts her wat❜ry zone, and sinks the poles.-FALCONER.

QUESTIONS.-1. What is the attraction of gravitation? 2. How is the tendency of bodies to fall produced? 3. What incident led Newton to the most wonderful discoveries? 4. How did he reason? 5. What is considered the mark of a superior genius? 6. What did Newton soon perceive respecting the force of gravity? 7. What did this lead him to conjecture? 8. How did he reason respecting the moon? 9. What has this principle of gravitation been styled? 10. What did Newton fully explain by it?

LESSON 17.

Centre of Gravity.

Perpendicularly, in the direction of a straight line up and down.
Pyramid, a pillar ending in a point.

THE centre of gravity of a body is that point about which all its parts, in any situation exactly balance each other, so that if a body be suspended or supported by this point, it will rest in any position. Whatever supports the centre of gravity bears the weight of the whole body; and while it is supported the body cannot fall. We may consider, therefore, the whole weight of a body as centered in this point. If a line is drawn from the centre of gravity of a body, perpendicularly to the horizon, it is called the line of direction; because it is the line which the centre of gravity would describe, if the body fell freely. The broader the base is upon which a body rests, the more difficult it will be to overturn it, as it must be moved the more to bring the line of direction beyond the base. A cask is easily rolled along, and so is a ball, but a box is moved with greater difficulty, When a box is longer than it is broad, it is much more easily turned on its side than set on its end. A building in the

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form of a pyramid is the most durable, because, as it becomes narrower and narrower as it ascends, each stone or brick is supported by those below. The pyramids of Egypt, both great and small, still remain, and without doubt will do so for thousands of years to come, while the vast temples are crumbling into ruin. In building, care is taken not to bring the upper rows of bricks beyond those below, and for this purpose a line and plummet are used. But it does not follow, because a building leans, that the centre of gravity does not fall within the base. There is a high tower at Pisa, a town in Italy, which leans fifteen feet out of a perpendicular direction; strangers tremble to pass by it, still it is found by experiment that the line of direction falls within the base, and therefore it will stand while its materials hold together.

The higher the centre of gravity is, the more easily may a body be overturned. Hence, a wagon or cart with a high load is more in danger of being overturned than one with a heavy load laid lower. This proves the injurious effect of rising in a coach or boat in danger of oversetting, the centre of gravity being thereby raised, and the line of direction thrown out of the base. In such circumstances the proper course is to lie down in the bottom, so as to bring the line of direction, and consequently the centre of gravity, within the base, and thus remove the danger of oversetting. Ropedancers perform astonishing feats by the assistance of a long pole with very weighty pieces of lead at each end, by which they balance themselves and recover firm footing, if likely to fall on either side. In our ordinary actions we regulate the motions of our bodies, as if we were most correctly studying the nature and effects of the centre of gravity. If a man wishes to rise from a chair, he throws his body forward. he is likely to fall on one side he leans to the other. A correct knowledge of the centre of gravity in bodies is of the utmost importance in the science of mechanics, as well as in many of the common actions of life.

If

QUESTIONS.-1 What is the centre of gravity? 2. The line of direction? 3. When does a body stand most firmly? 4. Why is a pyramid the most durable form of building? 5. What occasions a body to be easily overturned? 6. What is the proper course when a coach or boat is in danger of oversetting? 7. On what principle do we regulate our ordinary actions? 8. Show by fig. 12. the common centre of gravity of two bodies. 9. Illustrate by fig. 4. the overturning of a body, when the line of direction falls out of the base.