it, and if the air within be rarified enough, (as in all probability it will, because all the air, that is next to the water, is more rarified than other air, and this is more rarified than other air, that is next to the water,) I say, if the air within is rarified enough, it will not only be buoyed up to the surface of the water, causing a bubble there, but will leap clear out of the water, and will ascend in the air, till it is in equipoise with the circumambient air.

58. 1. I never yet could light of any satisfying reason, why the Heat of the Sun is so much greater near the Surface of the Earth, than at a distance from it. It is said that, near the Earth, the rays are doubled by reflexion. But they are not doubled; for none can think that the Earth reflects all the rays that fall upon it, at least not with as much strength as they come from the Sun, for the reflex light is nothing near so great as the direct light, it is very evident. But I suppose that the heat, that is very near the Earth, in a hot summer's day, is a hundred times greater than merely the direct rays would cause, instead of being only double. And I suppose, at three or four miles from the Earth, the heat is nothing to what it is very near: but there is as much of the reflex ray, to a very trifle, as we have close to the surface, for the rays, that are reflected from the Earth, do not cease, in going three or four miles, any more than the rays reflected from the Moon, or Venus, or Jupiter, or Saturn; and all the difference, otherwise, is only according to the squares of the distances from the centre; and what a small matter is that in three or four miles.

2. The heat therefore cannot be caused immediately, by the motion of the rays of the Sun, but also by the motion of other particles in the Atmosphere, set in motion by them. Now the reason, why particles should be much more set in motion, near the Surface of the Earth, than farther from it, we gave in 57; because that part of the Atmosphere, that is close to the Surface of the Earth, suffers much more from the rays, so that they will be much more heated, and rarified, and ascending; and the heavier, colder air, that is incumbent, getting under; so that it is the ascent of these agitated particles that chiefly causes the heat, which, by degrees, cooling as they ascend, being no longer subject to the violent force of the rays, will cause it to be much hotter near the surface and cooler at a dis


59. 1. The matter of the Tail of a Comet, does not ascend from the Sun, because it is made more rare than the Ether; for it must be very rare indeed to be so rare, that all the matter of the greatest tails might be contained in a nut-shell; (this is more rare than it is reasonable to suppose ;) but by the Comet's heating the Ether that is round about, so that the Ether will have a constant stream from the Comet upwards from the Sun, yea a very rapid stream, so as to carry some of the rarest parts of the Comet's Atmosphere with it.

2. These tails entirely cease to be emitted at a considerable distance from the Sun, not because the Comet wants heat to rarify, but because the Ether is so rare it is not strong enough to carry particles with it.

60. All Plants, from the beginning of the world, of the same kind, are nothing but so many branches of the first plant, or plants, proceeding ever since, and sprout out in exact order, and at regular distances. But this regularity consists in the equality of different periods. They do not continue to send forth branches, one after another, perpetually, without intermission; but this germination, has various stops, and stays, of equal lengths and distances, one from another. If the weather be never so suitable, the tree will not continue to emit branches continually, one after another, without intermission; but after such a number of branches are emitted, no more will sprout for some months, and then such a number again VOL. I.


will unfold themselves, ceasing again, at the due period. This may be ob served, at least in all the trees that grow in this climate, where are successions of summer and winter. These periods are usually suited to the length of these seasons. And once, when the heat here in New-England continued extraordinarily late, we have had part of two of these periods in a year; plants that had stopped, sprouted again. The twig grows, till the bud for the next year appears, and then ceases; but if the weather continues warm, it will be a considerable time before these buds will expand themselves. Another, and the largest distance is, from seed to seed. The fruit and seed is the extremity of a branch, and that branch or twig, from which the seed falls, never grows any more at all by it; the tree proceeds on no further that way.

61. It is the same thing that distant existence, distant as to place, should have influence on bodies, as in gravity; as that existence, distant as to time, being past, should have influence on their present existence, as in the successions of motion.

62. WIND. EXHALATIONS. It is certain, by experience, that winds do contribute so to drying up of moisture. It is not conceivable, that the wind should raise those bubbles, of which watery exhalations consist; but we conceive it, that it contributes to the raising of them, after this manner. It may contribute to the raising them from off the surface of bodies of water, by continually carrying off the moist air, and by bringing on that from the land which is dryer and more agile; whose parts are more briskly moved by the heat, and therefore, are more easily driven under the surface of the water, and so carry it off. It dries things that are moist upon the land, by carrying off the exhalations from the loose and porous parts, as soon as raised, so that they do not lodge again, and stand in the way of others.

[ocr errors]

63, PLANETS. A reason why the Greater Planets, as Jupiter, and Saturn, are placed at such a vast distance from the Sun, and the Lesser Planets nearer; is because, if such vast bodies were near, they would have abundantly greater influence by their attraction, to disturb the rest of the Sun, and so in time, to overthrow the whole system. The Comets would likewise be greatly exposed to their influence, and their orbits would be much disturbed by them. And it is fit they should be at a great distance from the Lesser Planets, otherwise they would greatly disorder their motion; and also from one another, for bodies of such mighty force and power must be kept at a distance, otherwise they will make dreadful work, one with another.

64. WAVES. Circular Waves in the water are begun, with a raising or depression of the water, in the centre of these circles, and are made thus. Suppose the water is raised into a hillock, at 1. This cannot be,

without the water subsiding in the space 2, to make the hillock; and the water at 3, is set in motion towards 1, to fill up that hollow. And it is most easy to conceive, that the water at 3, moving out of its place, and


thereby leaving a valley, the water at 4, will necessarily follow, and so on to 5, 6, 7, 8, 9, 10, 11:

2 3\4/5

6/78/9 10/11

for a valley being made at 2, the water at 3, comes to fill it up, and leaves a valley at 3; then the water at 4 moves inward, and leaves a valley there; so that the valley that began at 2, spreads around further and further, in a circle outward, from 1, 2, 3, 4, etc. successively. But we are to remember that the hillock of matter at 1, immediately subsides and flows every way into the space 2, and stops the water at 3, which was set in motion inwards; whereby it necessarily rises into a hill, for water, being stopped in its motion, will necessarily rise. The water at 3, being stopped and raised in an hill, this hill falling, contradicts and stops the water at 4, and causes that to rise; so that it is easy to conceive, why there must immediately succeed a rising, spreading after the same manner. We are to remember that we left a valley at 1, where the water subsided, and must necessarily subside lower than the ordinary surface; because when the hill was raised there, there was a valley round it, in the space 2. Now the weight and libration of the water, would cause all the water at 1, that was higher than that valley, to flow away. There being now, therefore, a valley at 1, and a hill at 2, it is easy to conceive, that the libration of the water, will raise another hill at 1, leaving a valley at 2, which valley will be communicated to 3, 4, 5, etc. after the same manner, and for the same reason, as the first valley; and this again will succeed another hill. And so there will be a continued succession of spreading hills and vallies, having their original in the successive hills and val lies, in the spaces 1, and 2, caused by the libration of the water.

But if we suppose, that in the first place, a valley, and not a hill, is made in the space 1, then the first circle will be an hill, and not a valley; for the water being expelled out of 1, necessarily thrusts up the water at 2, and

causes a hill there: the water at 2, subsiding, thrusts up the water at 3, that, the water at 4, and so on: and then there being the same reciprocation of the water at 1, and 2, as in the former case, causes the same succession of circular hills and vallies.

65. LEAVER.-PROBLEM. To give the reason, why the same force or weight, upon a Leaver or Balance, has a stronger or weaker influence. according as it is further from, or nearer to, the Center of motion. For instance, suppose that the weight D, hanging from the end A of the balance

[blocks in formation]



A B, is in equilibrio with the weight E, that is four times less, hanging at the other end, B, of the balance, at four times the distance from the center of motion, C. To solve this problem, we shall lay down the following Propositions, as most agreeable to the reason of man.

PROPOSITION 1. The same force or power, which, applied at a certain single moment, is sufficient to raise the greater weight, D, is requisite to raise the weight E, which is four times less, four times as far. This is evident, because the effect is just equal, and what is wanting in weight, in the lesser body, is exactly made up in the distance raised. If there is requisite a greater force, to raise the weight E four inches, than one, as there certainly is, for we suppose no continual repetition of the force, but an application of it for a certain moment: if so, then I say, there needs four times as much, for the weight resists the motion, as well while the body is moving the 2d moment, (space,) as the first, and as much the 3d, and 4th, as the 2d.

Hence we may learn, why the weight D will not sink, but hang in equilibrio with the lesser weight; because, if it subsided, it must raise the weight E, four times as much as it fell itself, every moment of its fall. But, in order to that, by the foregoing proposition, there would need a force snfficient to raise the weight D, that is, a force that is greater than the weight D. Wherefore, the weight D will not raise the weight E.

Coroll. It necessarily follows hence, that, if the weight E be made in the least greater, it will descend, for it hung in equilibrio before; but the reason of this will appear better, from the following Proposition.

PROPOSITION 2. The supporting or holding up of a greater weight, is an effect, that is fully equivalent to, and requires a force or power full as great, as the raising or carrying upwards a lesser weight. It is exceeding evident; because, if the least degree of force was added. it would carry upwards, even the greater weight. Or, we may take the Proposition in more general terms, thus: The bare resisting of a greater force, is equivalent to the carrying or moving a body against a lesser.

Hence it follows, that if the lesser body E be made in the least heavier, or removed further from the center of motion, it will subside, and raise the greater weight D; because, as has been shown already, it is not sufficient to raise it now; but if it was heavier, or further removed from A, the

supporting of it would require more force, than the raising it. Let the weight E be supposed to be removed to F, a fourth part of the former distance, and let the weight, at the same time, be made answerably lighter. It is evident, by the foregoing proposition, that it would then remain in equilibrio with the weight D. It is, therefore, evident by this, that if it be removed, without proportionally lessening its weight, it will sink, because the holding it up, would require greater force, than the raising it before.

66. SOUND. The cause of Sound is agreed to be, a vibrating, or a trembling motion of the air, consisting of quick and very sudden shocks, or leaps of the air, reciprocated. It is very easy to conceive, why the meeting of two hard bodies should cause such a trembling, correspondent to the trembling of these bodies; and why a body, moving steadily, though very swiftly, in the air, should cause but little such motion in the air. But we find, that the most violent sounds are caused by the shutting or closing of a gap or vacuity, that has been made in the air; and it is very agreeable to reason, that it should be so. For such a gap being made, it necessarily follows, from the weight of the incumbent atmosphere, and its very elastic nature, that the walls of the vacuity should rush together, with incredible violence, and that they should strike each other, with great force: upon which, the air that thus meets, will be very much compressed, and will again, by reason of its elasticity, very suddenly, and with violence, expand itself again; and, according to the laws of the motion of elastic bodies, with twice as great violence, as the like quantity of air, compressed in a solid body, would expand itself: for then, the air would be beaten back, only by its own spring, but now, by that, and also by the spring of the air that it met. Let the air meet at the plane A B, and be by the shock much compressed. It is evident, that the air, on the side of that plane C, will not D only leap back towards that side, by its own elasticity, but that the elasticity of the air on the other side of the plane, the side D, will impel it towards the side C, with equal force. And so the reciprocation will be repeated, with great violence, for a time. Sound, that is made by the collision of solid bodies, is not made by the " sudden start of the air, from between the closing parts of those bodies; but the vibration of the air is begotten by a vibration of the parts of the bodies themselves; for, if the body that is smitten be set upon another, the sound will be like that of the body it stands upon; which can be for no other reason, than that the vibration is communicated to the parts of that body, and from them to the air. So, from the communication of sound, in a long stick of timber, if we lay our ear at the farther end, when it is struck, the sound will seem to be made there; which is doubtless, from the communication of the vibration, through the parts of the timber.




The loudness of many sounds, doubtless arises from the continualness of them that is, let pulses of the same degrée be continued, or constantly repeated in the air, and on the organ, every successive moment: the Sound will not only be longer, but abundantly louder, than if only one of these pulses smote the organ, and ceased; that is, provided those pulses are repeated so quick, that the impression, made upon the organ by one pulse, does not cease, till another comes, or so quick, that several of them smite the organ, before the mind can perceive any succession, or while one idea remains unvaried in the mind, before it has time to grow old, or perish, in any degree. The reason, in both cases, is very plain; for if the impression of one pulse remains upon the organ, till another comes, the new impression being added to the old, the whole impression must be

« ElőzőTovább »