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dominates over that of every other body. The houses are riveted to the earth so strongly by their gravity, that neither the other houses nor the mountain can draw them away. That the attraction of a mountain, however, may affect the perpendicular attraction of the earth, has actually been found by experience. The weight of a plumb-line held in the hand on the declivity of a high mountain, has been found to incline a little towards the mountain. Here, slight as the attraction of the mountain is, compared to that of the earth, yet, as the weight suspended is so much nearer the mountain than the earth's centre, the attraction of the mountain has a slight influence; so slight, indeed, that it requires very nice instruments to ascertain its effect. If the earth attracts all bodies near its surface, it may further be asked why the atmospheric air does not, like other bodies, fall to the ground? The truth is, that it does so fall. Its lowest stratum or layer is in contact with the earth, and each inferior stratum supports that which is above it. From the pressure of the superior strata upon the inferior, the air near the surface of the earth is much denser than that of the higher regions. It may also be objected, that smoke, steam, air-balloons, &c. which ascend through the air, in place of falling to the ground, are exceptions to the general law of nature. But, in truth, these phenomena, when rightly understood, are in perfect accordance with it. If you throw a bit of cork into a tub of water, it immediately rises again to the surface; and, if you pour more water into the tub, the cork will rise still higher. The reason is obvious. The water is denser than the cork, and is therefore more strongly attracted towards the earth; and, because two bodies cannot, on account of their impenetrability (as you have already seen), occupy the same place at the same time, the water in descending displaces the cork, and forces it upwards in order to make way for itself. For the like reason, smoke and steam, and every vapour which is lighter than the surrounding atmosphere, are forced by the descent of the heavier air, to rise to a higher region, where the atmosphere is of equal density with themselves. In the same manner, Mr Green's aerial voyage from the neigh

bourhood of Leith, which many of you witnessed, was effected by the air enclosed in his balloon being so much lighter than the atmosphere in the lower regions. You will thus understand, that a body lighter than the surrounding air ascends; that one of equal density remains suspended in it; and one of greater density falls through it. Even the falling body, however, encounters considerable resistance or obstruction from the air in its descent. If you throw a stone into a tub of water, it will fall more slowly, than if the water were taken out of the tub; and if the air, as well as the water, were taken out of the vessel, the stone would descend more rapidly still. This resistance of the air is in proportion to the surface of the body exposed to the resistance. A sheet of paper will fall much more quickly, when wrapt up into a ball, than when its whole surface is exposed to the air; and a bit of gold will fall much more slowly when beaten out into a leaf, than while it continues in its original mass. A cannon-ball of iron will fall more rapidly to the ground, than a ball of leather of the same dimensions; because the latter is more affected by the resistance of the air, in proportion to its quantity of matter. If the air were removed, the iron ball and the leather ball would fall to the ground with precisely the same velocity. To this it may be objected, that the ball of iron, on account of its greater quantity of matter, is acted upon with more force than the leather ball. But then, it ought to be remembered, on the other hand, that, for the same reason, the former will require a greater force to move it. A beautiful experiment in illustration of this is exhibited in the Natural Philosophy class. The air having been previously pumped out of a tall glass vessel called a receiver, a guinea and a light feather, let slip at the same time at the top of the vessel, fall at one and the same moment to the bottom of it. A simpler experiment you may try for yourselves: a piece of paper of the same size and figure with a penny-piece will, of course, fall much more slowly through the air than the copper; but if you lay the paper close upon the piece, so long as little air intervenes between them, they will continue to fall together. There is in every solid body a

point, called the centre of gravity, about which all the parts exactly balance each other. If this point be supported the body will be steady; if not it will fall till it is supported. Thus let the line AB represent a table, and the figure CDEF a box; the

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the outside of the figure representing the box. The reason why a child or a drunk man falls is, that he does not keep his centre of gravity supported; or, in other words, because the line of direction falls without his body, instead of between his feet. All the art of a rope-dancer, in the same manner, consists in supporting his centre of gravity. It is for the same reason, that a vessel with a narrow base is so easily upset: because, if it be inclined ever so little to one side, its centre of gravity is no longer supported. You will now also perceive the reason why a ball rolls down a slope, while a square body only slides down. The ball can touch the declivity only in a single point, and, as that point is not in the line of direction, the centre of gravity is not supported. Where every part of a body is of equal density, the middle of the body, which is called the centre of magnitude, is also the centre of attraction. But, because one part of a body is sometimes made of heavier material than another, the centre of magnitude is not always the centre of gravity. Hence, by putting a heavy substance in part of a body, the rest of which is composed of lighter material, many entertaining experiments are shown, in which bodies refuse to remain at rest in what would appear to be their natural position. Thus also, by putting a bit of lead into the side of a cylinder of wood, the centre of gravity, in descending, will make the cylinder itself ascend a declivity. The centre of gravity is not always in the body, but is

sometimes in empty space. Thus the centre of gravity of a ring is the centre of the space which the ring encloses. If a body be suspended by a point in the line of direction, it will remain stationary; but it can rest in no other position, as you may perceive by suspending a piece of pasteboard by one of its corners. When two bodies are fastened together by a bar, or string, or any power whatever, they are, in this matter, to be considered as one body, having a common centre of gravity. If the two bodies be of equal weight, the centre of gravity is in the middle of the line which unites them. If one be heavier than the other, the centre of gravity is proportionally nearer the heavy body. Some other most important circumstances connected with gravitation remain to be noticed in illustration of the laws of motion.

LAWS OF MOTION.

MOTION, as every one knows, consists in a change of place. It depends upon a variety of circumstances. 1. From what was formerly said with regard to the inertia of matter, it appears that no body begins to move, except through the operation of some power, which puts it in motion. This moving power, whether it be animate or inanimate, attractive or repulsive, is called force. Thus, in playing hand-ball, the blow given by the hand is the force which impels the ball; the pulling of the horse is the force which draws a carriage; the particles of matter are drawn together by the force of cohesion, and they are separated by the force of heat.-2. When a body is acted upon by a single force, its motion, as might be expected, is always in a straight line, and in the direction of the force which moves it.-3. The velocity with which a body moves (or, in other words, the distance which it goes in a given time), is always in proportion to the force which put it in motion. Thus, if of two bodies one goes eight miles an hour, while the other goes only four, the velocity of the former motion is double that of the latter, and is occasioned by the operation of a double force.

-4. Where a body is set in motion by the exertion of a single force, which instantly ceases, the motion of the body is uniform (or, in other words, the body moves throughout the whole of its course with the same velocity); and, if unobstructed, this motion will continue for ever. It is very true, that a stone, rolled along the ground by one impulse of the hand, goes every moment more and more slowly, until it at length stops altogether. But then, it will be remembered, that the stone, besides being exposed to the friction of the earth, and the resistance of the air, is every moment acted upon by the force of gravitation. It may perhaps be thought, that the duration of the motion will depend upon the strength or weakness of the moving force. This, however, is quite a mistake. If a body receive only a gentle impulse, its motion (as we have seen) will be slow, but this slow motion, unless counteracted by some other force, will continue for ever.5. The momentum of a moving body (that is to say, the force with which a body in motion acts upon another body,) depends upon two circumstances; namely, the quantity of matter or weight, and the quantity of motion or velocity, of the moving body. Every one knows by experience that the heavier any body is, the greater is its force; but, by increasing the velocity of a lighter body, you may render its momentum much greater than that of a heavier one. Thus, an arrow shot from a bow has a greater momentum, than a stone thrown by the hand. Upon this principle, though you may place a pound-weight upon a china plate, without doing it the slightest injury, yet, if you let the weight fall from the height of only a few inches, it will, in consequence of the velocity which it has thus acquired, dash the china to pieces. If you let a pound-weight fall upon the floor from the height only of an inch and a quarter, it will strike the floor with a momentum equal to twice its weight. From what has been said you will see, that, in order to ascertain the momentum of a body, you must multiply the weight by the velocity. Thus, the momentum of a body of two pounds weight, moving at the rate of 16 feet in a second, is said to be 32, because 2 multiplied into 16 gives 32; the momentum of a body

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