force of projection, because the air alone, presents less impediment than the air and ice, and there is now nothing to oppose its constant motion, except the resistance of the air, and its own weight, or gravity.

If the air be exhausted, or pumped out of a vessel by means of an air pump, and a common top, with a small, hard point, be set in motion in it, the top will continue to spin for hours, because the air does not resist its motion. A pendulum, set in motion, in an exhausted vessel, will continue to swing, without the help of clock work, for a whole day, because there is nothing to resist its perpetual motion, but the small friction at the point where it is suspended.

We see, then, that it is the resistance of the air, of friction, and of gravity, which cause bodies once in motion to cease moving, or come to rest, and that dead matter of itself, is equally incapable of causing its own motion, or its own

rest.

We have perpetual examples of the truth of this doctrine, in the moon, and other planets. These vast bodies move through spaces which are void of the obstacles of air and friction, and their motions are the same that they were thousands of years ago, or at the beginning of creation.

Attraction. By attraction is meant that property, or quality in the particles of bodies, which make them tend toward each other.

We know that substances are composed of small atoms, or particles, of matter, and that it is a collection of these, united together, that forms all the objects with which we are acquainted. Now, when we come to divide, or separate any substance into parts, we do not find that its particles have been united, or kept together by glue, little nails, or any such mechanical ineans, but that they cling together by some power, not obvious to our senses. This power we call attraction, but of its nature or cause, we are entirely ignorant. Experiment and observation, however, demonstrate, that this power pervades all material things, and that under different modifications, it

Why, with the same force of projection, will a ball move farther through the air than on the ice? Why will a top spin, or a pendulum swing longer, in an exhausted vessel than in the air? What are the causes which resist the perpetual motion of bodies? Where have we an example of continued motion, without the existence of air and friction? What is meant by attraction? What is known about the cause of attraction? Is attraction common to all kinds of matter, or not?

not only makes the particles of bodies adhere to each other, but is the cause which keeps the planets in their orbits as they pass through the heavens,

Attraction has received different names, according to the circumstances under which it acts.

The force which keeps the particles of matter together, to form boles, or masses, is called attraction of cohesion. That which inclines different masses towards each other, is called attraction of gravitation. That which causes liquids to rise in tubes, is called capillary attraction. That which forces the particles of substances of different kinds to unite, is known under the name of chemical attraction. That which causes the needle to point constantly towards the poles of the earth is magnetic attraction; and that which is excited by friction in certain substances, is known by the name of electrical attraction.

The following illustrations, it is hoped, will make each kind of attraction distinct and obvious to the mind of the student. Attraction of cohesion acts only at insensible distances, as when the particles of bodies apparently touch each other.

Take two pieces of lead, of a round form, an inch in diameter, and two inches long; flatten one end of each, and make through it an eye-hole for a string. Make the other ends of each as smooth as possible, by cutting them with a sharp knife. If now the smooth surfaces be brought together, with a slight turning pressure, they will adhere with such force that two men can hardly pull them apart by the two strings.

In like manner, two pieces of plate glass, when their surfaces are cleaned from dust, and they are pressed together, will adhere with considerable force. Other smooth substances present the same phenomena.

This kind of attraction is much stronger in some bodies chan in others. Thus, it is stronger in the metals than in most other substances, and in some of the metals it is stronger than in others. In general, it is most powerful among the particles of solid bodies, weaker among those of liquids, and

What effect does this power have upon the planets? Why has attracion received different names? How many kinds of attraction are there? How does the attraction of cohesion operate? What is meant by attraction of gravitation? What by capillary attraction? What by chemical attraction? What is that which makes the needle point towards the pole? How is electrical attraction excited? Give an example of cohesive attraction.

probably entirely wanting among elastic fluids, such as air, and

the gases,

Thus, a small iron wire will hold a suspended weight of many pounds, without having its particles separated; the particles of water are divided by a very small force, while those of air, are still more easily moved among each other. (These different properties depend on the force of cohesion with which the several particles of these bodies are united.

When the particles of fluids are left to arrange themselves according to the laws of attraction, the bodies which they compose assume the form of a globe or ball.

Drops of water thrown on an oiled surface or on wax-globules of mercury,-hail stones,— —a drop of water adhering to the end of the finger,-tears running down the cheeks, and dew drops on the leaves of plants, are all examples of this law of attraction. The manufacture of shot is also a striking illustration. The lead is melted and poured into a sieve, at the height of about two hundred feet from the ground. stream of lead immediately after leaving the sieve, separates into round glebules, which, before they reach the ground, are cooled and become solid, and thus are formed the shot used by sportsmen.

The

To account for the globular form in all these cases, we have only to consider that the particles of matter are mutually attracted towards a common centre, and in liquids being free to move, they arrange themselves accordingly.)

In all figures except the globe, or ball, some of the particles must be nearer the centre than others. But in a body that is perfectly round, every part of the outside is exactly at the same distance from the centre.

Thus the corners of a cube, or square, are
at much greater distances from the centre,
than the sides, while the circumference of
a circle or ball is every where at the same
distance from it. This difference is shown
by fig. 1, and it is quite obvious, that if the
particles of matter are equally attracted to-
wards the common centre, and are free to
arrange
themselves, no other figure could

In what substances is cohesive attraction the strongest? In what substances is it weakest? Why are the particles of fluids more easily separated than those of solids?

possibly be formed, since then every part of the outside is equally attracted.

The sun, earth, moon, and indeed all the heavenly bodies, are illustrations of this law, and therefore were probably in so soft a state when first formed, as to allow their particles freely to arrange themselves accordingly.

Attraction of gravitation. As the attraction of cohesion unites the particles of matter into masses or bodies, so the attraction of gravitation tends to force these masses towards each other, to form those of still greater dimensions. The term gravitation, does not here strictly refer to the weight of bodies, but to the attraction of the masses of matter towards each other, whether downwards, upwards, or horizontally. The attraction of gravitation is mutual, since all bodies not only attract other bodies, but are themselves attracted.

Fig. 2.

Two cannon balls, when suspended by long cords, so as to hang quite near each other, are found to exert a mutual attraction, so that neither of the cords is exactly perpendicular, but they approach each other, as in fig. 2.

In the same manner, the heavenly bodies, when they approach each other, are drawn out of the line of their paths, or orbits, by mutual at

traction.

The force of attraction increases in proportion as bodies approach each other, and by the same law it must diminish in proportion as they recede from each other.

Attraction, in technical language, is inversely as the squares of the distances between the two bodies. That is, in proportion as the square of

the distance increases, in the same proportion attraction decreases, and so the contrary, Thus, if at the distance of 2 feet, the attraction be equal to 4 pounds, at the

What form do fluids take, when their particles are left to their own arrangement? Give examples of this law. How is the globular form which liquids assume, accounted for? If the particles of a body are free to move, and are equally attracted towards the centre, what must be its figure? Why must the figure be a globe? What great natural bodies are examples of this law? What is meant by attraction of gravitation? Can one body attract another without being itself attracted? How is it proved that bodies attract each other? By what law, or rule, does the force of attraction increase?

distance of 4 feet, it will be only 1 pound; for the square of 2 is 4, and the square of 4 is 16, which is 4 times the square of 2/ On the contrary, if the attraction at the distance of 6 feet be 3 pounds, at the distance of 2 feet it will be 9 times as much, or 27 pounds, because 36, the square of 6, is equal to 9 times 4, the square of 2.

The intensity of light is found to increase and diminish in the same proportion. Thus, if a board a foot square, be placed at the distance of one foot from candle, it will be found to hide the light from another board of two feet square, at the distance of two feet from the candle. Now a board of two feet square is just four times as large as one of one foot square, and therefore the light at double the distance being spread over 4 times the surface, has only one fourth the intensity,

one; B being four times the size of A.

This experiment may be easily tried, or may be readily understood by fig 3, where c represents the candle, A the small board, and B the large

The force of the attraction of gravitation, is in proportion to the quantity of matter the attracting body contains.

Some bodies of the same bulk contain a much greater quantity of matter than others thus, a piece of lead contains about twelve times as much matter as a piece of cork of the same dimensions, and therefore a piece of lead of any given size, and a piece of cork twelve times as large, will attract each other equally.

Capillary Attraction. The force by which small tubes, o porous substances, raise liquids above their levels, is called capillary attraction.

If a small glass tube be placed in water, the water on the inside will be raised above the level of that on the outside of

Give an example of this rule. How is it shown that the intensity of light increases and diminishes in the same proportion as the attraction of matter? Do bodies attract in proportion to bulk, or quantity of matter? What would be the difference of attraction between a cubic inch of lead, and a cubic inch of cork? Why would there be so much difference? What is meant by capillary attraction? How is this kind of attraction illustrated with a glass tube?