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greater. And if many pulses affect the mind, before the mind can per ceive any succession, and during the time that one idea or mental impression remains unvaried in the mind, then there will be the addition of several impressions together, which must be stronger than one alone. If three sounds, or three pulses, be made upon the air and ear, in so little time, that the mind has not the least sense of succession, and they seem to be all perfectly at once; then it will be all one to the mind, as if these three sounds had been made really at once; and the sound will be as much louder than one of these sounds alone, as three, joined together, would be louder than one of them.

Coroll. 1. The shrillness of the sound of a bell arises, very much, from this cause. There is a continuance of pulses, exceedingly quick, repeated one after another, answerable to the vibrations of the metal; and perhaps one of these vibrations, singly, would not make a louder noise, than a rap with a staff, upon a piece of wood, which yet, is not the one hundredth part so loud, as the rigning of a bell.

Coroll. 2. The loudness of Thunder arises, also, very much, from hence; for the Lightning, that breaks forth from the cloud, and comes so instantaneously down to the Earth, smites the air successively, all the way. And if Sound came as quick as the light, the Sound would all seem to be together, in a moment; but because the places, from whence the Sound comes, are gradually further and further off, and so the Sound comes to us successively; but not so slow, but that the stroke of the Lightning upon the air, for a long space, seems to come to us at once. It may be, that, in the loudest claps of Thunder, if we only received the impression that the Lightning made in going one foot, and were not reached by the impression made in the rest of its course, it would not be a quarter so loud, as the report of a pistol.

67. THUNDER. It is remarkable of Thunder, how long one part of the sound will be heard after another, when it is evident that the sound is made all in an instant, by the Lightning, which continues no longer. This arises from the length of the stream of Lightning, whereby one part is a great deal farther from us than another, so that the sound is a great while coming successively. Hence it is, that in claps of thunder, that are near us, the first noise that we hear seems to be very near the Earth, and then it seems to go further and further from us, and the last will be a murmuring up in the clouds; for although the noise that was made in the clouds, and the noise near the earth, was made together as at an instant, yet that in the clouds is much farther, and therefore is longer coming, and is a much lower sound when it sounds.

The rapid vibration of the air jars and jumbles, breaks and condenses, the bubbles of the cloud; whence it is, that, soon after hard claps of thunder, rain falls in greater plenty.

I regard Thunder as a meteor by far the most wonderful and least explicable of any whatsoever. But that we may make some approaches to the knowledge of the true nature of it, we shall lay down these following propositions.

1. The Streams of Lightning are not caused by any solid burning, or red-hot mass of matter, exploded with such swiftness as to cause it to appear as if there were one continued stream of light; nor are the effects of Lightning caused by the violent stroke of any such solid mass. For if Lightning were such a body projected, it would be projected according to the laws of projected bodies; whereas the path of the Lightning is exceedingly far from it, being very crooked and angled. If Lightning were a

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solid body, projected from the cloud at A, towards E, with such a prodigious celerity, it proceeds according to the direction A, very nearly, and turns short: at E in the free air, and so at F, B and C; for, when it is projected with such a prodigious force, it must also be a prodigious force, that must change the course, of it so short, and not the force of the free and yielding air.-But if any should suppose, that the change of the course of the Lightning might be caused, by some very violent eruptions of fire, at these angles, where the course is changed, that gives the thunderbolt a new projection:-to this I reply, that the fiery stream of Lightning is smooth and even; but if there were any such new eruptions, they would be seen by a sudden and extraordinary expansion of the light, in those places. But what proves, that this cannot be the reason of the crookedness of the path of the Lightning, is, that, as the flash of the Lightning is repeated once or twice, however crooked and angled the path is, yet it is every time the same: a stream of Lightning darts from the clouds two or three times over, and every time exactly in the same path. And sometimes there is a continued stream, for some time, with a tremulous motion. Now if these repeated flashes were one bolt exploded after another, and the reason of the Lightning's changing its course were new eruptions of fire, how should every bolt proceed, so exactly, in the same path. And further, the effects of Lightning, upon earthly bodies, can in no wise be accounted for, by the violent projection of a solid mass, and do plainly show that they are not produced by such a cause. There is no such effect, as is caused by the explosion of a cannon ball. It is not worth while to stand to particularize, for it is exceedingly evident that none of the effects of Lightning arise from any such cause. Nor

2. Are those streaks of Lightning caused by a vein of combustible matter's taking fire, and the fire's running from one end of the vein to the other almost instantaneously. This would not produce any of those effects, which are caused by lightning, except we should suppose that these veins enter into the hearts of trees, rocks, and metals, and bodies of animals. If it were, it would be a wonder that the lower ends of these veins never took fire from fires that are upon earth. But

3. Lightning seems to be this: An almost infinitely fine, combustible matter, that floats in the air, that takes fire by a sudden and mighty fermentation, that is some way promoted by the cool and moisture, and perhaps attraction, of the clouds. By this sudden agitation, this fine, floating matter, is driven forth with a mighty force one way or other, which ever way it is directed, by the circumstances and temperature of the circumjacent air; for cold and heat, density and rarity, moisture and dryness, has almost an infinitely strong influence upon the fine particles of matter. This fluid matter, thus projected, still fermenting to the same degree, divides the air as it goes, and every moment receives a new impulse by the continued fermentation; and as its motion received its direction, at first, from the different temperature of the air, on different sides, so its direction is changed, according to the temperature of the air it meets with, which renders the path of the lightning so crooked. The parts are so

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fine, and are so vehemently urged on, that they instantaneously make their way into the pores of earthly bodies, still burning with a prodigious heat, and so instantly rarifying the rarifiable parts. Sometimes these bodies are somewhat bruised; which is chiefly by the beating of the air that is, with great violence, driven every way by the inflamed matter.

68. GRAVITY. If there be any thing, that makes us prone to seek for a farther cause of Gravity than Solidity, it is because Solidity is a quality so primary, that the very being of the thing depends on it. If we remove the idea of Solidity, there remains nothing at all; but we can con. ceive of something existing without thinking of gravitating at a distance. They are both of them essential and primary qualities: but there is this difference-the one is essential in order to the very existence, the other in order to the harmonious existence of body. Though Gravity itself, between the continuous parts, is necessary in order to the existence, the mind does not so intuitively see how. But Gravity is a quality more primary in these respects, and more essential than Mobility is, which none seek a reason for, or in the least question to be a primary property of matter.

69. DENSITY. PORES. A Body, which is very hard, may not have the thonsandth part of the space contained within its bounds, filled with matter, though we should not suppose that the parts of the body had a particular disposition contrived for this end. We need merely suppose the primogenial atoms to be of all manner of figures, indifferently and accidentally cast together in a heap. If so, we may suppose fairly, that this heap will not be above half of it matter. Let these heaps constitute so many particles of all figures indifferently, and yet consistent and solid enough, for aught we know. Let these particles be cast together to constitute other particles, they also will leave half the space empty, even of them, so that half the space between these particles will be empty, and half within them, so that only a quarter will be full. If we suppose other particles to be made of these again, but an eighth part will he full. And by the time we have had ten such compositions we shall not have the thousandth part of the space filled.-[N. B. This has been thought of before.]

70. ELASTICITY may be explained after a yet different manner, than by the violent motion of the particles, and I forsee must be. And first I shall show, that it may be differently explained; and secondly, that although this intestine motion may be, and doubtless often is, a secondary cause of Elasticity, yet that it cannot be the first foundation of it, but that this motion itself must be explained from another Elasticity.

1. The Attraction of particles to other particles which they touch, or to which they are very near, may eause, and indeed cannot but cause, Elasticity. For if the touching particles tend exceeding strongly to each other, as is most certain they do; then, if they are in the least separated, unless so far as to be out of the strength of the attraction, they will very strougly tend to move to each other to touch again; so that, if by any force they are a little pulled asunder, if the force that holds them asunder be taken away, they will immediately with great violence rush together again, and that in most bodies, whose particles are strongly united together after such a separation, they will with great force recover themselves.

And here I would take notice of two things, that pretty much depend on each other. (1.) That the particles of a rare body, by this way of explaining their union, may be much more strongly united than a denser one; for the strength of the union consists in the opposite position of the surface of the particles to each other; but yet there may be a great many particles

in a little room, and yet the surfaces not lie opposite one to another. For matter, of the quantity of a foot cube, may be so rarified, as to be extended as big as the Universe, and yet there shall not be one hair's breadth, but what has some of that matter in it, and yet the body shall be perfectly hard, and no part of it moveable, by less than Infinite strength. For it may be drawn out, to such a fine wire, that shall be a continued, uninterrupted, absolute plenum, so folded, coiled and tangled, within itself, and running every way backwards and forwards, as that not a part of space, so big as a ray of light, shall be without some of it, and yet it shall be what we call an Atom, and the continuity or touching by planes shall be uninterrupted. (2.) That the Particles or Atoms of bodies may be condensed, or thrust one in amongst another, and yet the union of the Atoms shall not be much the stricter; because Atoms, being infinitely hard bodies, their protuberances infinitely hard, and their surfaces unalterable, they may be jammed in, one amongst another, and yet their surfaces not adhere much more strictly one to another. And further, this perfect hardness of the Atoms, may hinder their being thrust in one amongst another; but will not hinder their being pulled asunder.

Now let the body A B C D, whose particles have a firm union, be bent towards D. It is evident, either that the particles at B are pulled farther, one from another, than they were; or that the particles at D are more condensed together; or both. But, as we have said, the particles at D will not be so easily thrust nearer together, because of their stubborn nature, as the particles at B may be drawn B asunder. It is also evident, that the particles at B, that are drawn asunder, by tending to come together again, will tend to pull the ends of the body back again to, and to recover it to, its former straitness: which tendency is Elasticity. And further, it is probable, by what we have said already, that if the particles at D, are thrust together, their tendency to each other will not be very much increased, so as to hold the body in its bent posture, as the particles at B tend to pull it strait. And however that is, there is no need to suppose that the particles at D are thrust nearer together; and it is probable, the constitution of firm bodies hinders it. And certainly the constitution may be such as to hinder it, much more than to hinder the pulling of bodies asunder; for it is certain that Atoms, being infinitely hard, if they touch only in their prominences, cannot be made to touch nearer, but may be drawn asunder. Wherefore it is certain, that Elasticity may be caused by this means.

2. The second thing to be proved was, That a rapid motion cannot be the first original of Elasticity. For, if we suppose that those particles, which are shut up in a little room, are continually in a violent motion, and rebound from side to side, yet Elasticity is necessary, in order to the maintaining of the motion. Otherwise, at the first stroke against the walls of the room, it would lose all its motion. There is necessary, therefore, another Elasticity, in order to the maintaining of this motion; which, being maintained by this first original Elasticity, may, in the aforesaid manner, cause Elasticity, as undoubtedly it does in the Atmosphere. Therefore, we see that its motion being increased, its elasticity is increased with it. Here we would note, that we think no phenomena contradict what we have said of Elasticity, arising from the tendency of separated atoms, to reunion with their fellows for instance, that of the hammer and anvil. The hammer does not thrust the atoms quite from their fellows; if they did so, a bruise would ensue; but the more it bruises, the less it rebounds. It does but just open and separete their surfaces, but not to so great a distance, VOL. I

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but that they immediately close again, after the violence of the stroke is over. And so it is in the former instance. Though the particles of the body A B C D, at D, seem to be thrust together, yet they are not quite put out of their natural place, not so much but that their strong tendency to their fellows, immediately brings them to the same places again. If they are quite dispossessed of their places, the body will not recover itself again, as we see if the body be too much bent. So that, what alteration there is, among the particles at D, may help the Elasticity, as well as the attractions of the particles at B.

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A body, whose particles are firmly united, cannot be smitten by another body, so as to make an impression upon it, but that the particles near the surface, where the impression is made, will have their surface drawn from each other. For instance, let A B C D, be the range of particles of a body, that before was round, but has an impression made by the stroke of another body at B. It is evident that the particles at B, will gape even inwards, as in the figure, and the particles at A, and C, will gape outwards; and that by their mutual attraction, they will recover themselves again, and thereby bring the body A B C D, to its former roundness; which will cast back the body that struck it, with the same celerity, as its surface at B recovers itself.

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Now it is probable, that rays of light are particularly formed, by the curious hand that made them, for this Elasticity.

71. ABYSS. It is undoubted, that there is a vast Abyss of water under us, above which, the surface of the Earth is stretched forth, and on which it rests; and it must undoubtedly be heavier, than the matter of the upper shell. Undoubtedly, also, the springs and fountains are much caused, by "the ascent of this water, in the chinks of the ground, streaming up by virtue of the central heat, and therefore, that there is a communication between the Abyss and the Sea. But if it be, by its own nature, heavier than the Earth, it is inconceivable how it should become lighter, when it has ascended to the surface, and is condensed in springs, and even as light as other water. And if it could be so, how should any of this water ever return to mix with the water of the Abyss again, by any communication that the sea has with it, or any of the sea water, in the room of it? For the great difference in the specific gravity, will forever hinder any mixtion or com nunication; and at this rate, the Abyss would in time be exhausted of its dense and heavy fluid, and filled with lighter in the room of it, or the world would be overflowed by a second Deluge.

water.

There is no other way, therefore, than that this water, when it is in the Abyss, in consequence of pressure, is heavier than earth; but when it is upon the surface, and the pressure is removed, it becomes lighter, as other And when it returns there again, or the water of the sea in the room of it, it becomes as heavy as it was before: which can be no otherwise than by compression. And if water be a body, that is capable of any compression by any means, doubtless it is compressed by that prodigious force to which it is subjected, by the weight of a body, of water, of four or five hundred miles thickness, incumbent upon it. If we cannot compress water but very little, or not at all, it is certainly merely for want of strength; for all compounded bodies, that have not an absolute plenitude, are undoubtedly capable of compression; their particles can be squeezed

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