2. Now we call those bodies heavy, which, unless they be hindered by some force, are carried towards the centre of the earth, and that by their own accord, for aught we can by sense perceive to the contrary. Some philosophers therefore have been of opinion, that the descent of heavy bodies proceeded from some internal appetite, by which, when they were cast upwards, they descended again, as moved by themselves, to such place as was agreeable to their nature. Others thought they were attracted by the earth. To the former I cannot assent, because I think I have already clearly enough demonstrated that there can be no beginning of motion, but from an external and moved body; and consequently, that whatsoever hath motion or endeavour towards any place, will always move or endeavour towards that same place, unless it be hindered by the reaction of some external body. Heavy bodies, therefore, being once cast upwards, cannot be cast down again but by external motion. Besides, seeing inanimate bodies have no appetite at all, it is ridiculous to think that by their own innate appetite they should, to preserve themselves, not understanding what preserves them, forsake the place they are in, and transfer themselves to another place; whereas man, who hath both appetite and understanding, cannot, for the preservation of his own life, raise himself by leaping above three or four feet from the ground. Lastly, to attribute to created bodies the power to move themselves, what is it else than to say that there be creatures which have no dependance upon the Creator? To the latter, who attribute the descent of heavy bodies to the attraction of the earth, I assent. But by what motion this is done, hath not as yet been explained by any man. I shall therefore in this place say somewhat of the manner and of the way by which the earth by its action attracteth heavy bodies.

3. That by the supposition of simple motion in the sun, homogeneous bodies are congregated and heterogeneous dissipated, has already been demonstrated in the 5th article of chapter XXI. I have also supposed, that there are intermingled with the pure air certain little bodies, or, as others call them, atoms; which by reason of their extreme smallness are invisible, and differing from one another in consistence, figure, motion, and magnitude; from whence it comes to pass that some of them are congregated to the earth, others to other planets, and others are carried up and down in the spaces between. And seeing those, which are carried to the earth, differ from one another in figure, motion, and magnitude, they will fall upon the earth, some with greater, others with less impetus. And seeing also that we compute the several degrees of gravity no otherwise than by this their falling upon the earth with greater or less impetus; it follows, that we conclude those to be the more heavy that have the greater impetus, and those to be less heavy that have the less impetus. Our inquiry therefore must be, by what means it may come to pass, that of bodies, which descend from above to the earth, some are carried with greater, others with less impetus; that is to say, some are more heavy than others. We must also inquire, by what means such bodies, as settle upon the earth, may by the earth itself be forced to ascend.

4. Let the circle made upon the centre C (in fig. 2) be a great circle in the superficies of the earth, passing through the points A and B. Also let any heavy body, as the stone A D, be placed anywhere in the plane of the equator; and let it be conceived to be cast up from A D perpendicularly, or to be carried in any other line to E, and supposed to rest there. Therefore, how much space soever the stone took up in A D, so much space it takes up now in E. And because all place is supposed to be full, the space A D will be filled by the air which flows into it first from the nearest places of the earth, and afterwards successively from more remote places. Upon the centre C let a circle be understood to be drawn through E; and let the plane space, which is between the superficies of the earth and that circle, be divided into plane orbs equal and concentric; of which let that be the first, which is contained by the two perimeters that pass through A and D. Whilst therefore the air, which is in the first orb, filleth the place A D, the orb itself is made so much less, and consequently its latitude is less than the strait line A D. Wherefore there will necessarily descend so much air from the orb next above. In like manner, for the same cause, there will also be a descent of air from the orb next above that; and so by succession from the orb in which the stone is at rest in E. Either therefore the stone itself, or so much air, will descend. And seeing air is by the diurnal revolution of the earth more easily thrust away than the stone, the air, which is in the orb that contains the stone, will be forced further upwards than the stone. But this, without the admission of vacuum, cannot be, unless so much air descend to E from the place next above; which being done, the stone will be thrust downwards. By this means therefore the stone now receives the beginning of its descent, that is to say, of its gravity. Furthermore, whatsoever is once moved, will be moved continually (as hath been shown in the 19th article of chapter VIII) in the same way, and with the same celerity, except it be retarded or accelerated by some external movent. Now the air, which is the only body that is interposed between the earth A and the stone above it E, will have the same action in every point of the strait line E A, which it hath in E. But it depressed the stone in E; and therefore also it will depress it equally in every point of the strait line E A. Wherefore the stone will descend from E to A with accelerated motion. The possible cause therefore of the descent of heavy bodies under the equator, is the diurnal motion of the earth. And the same demonstration will serve, if the stone be placed in the plane of any other circle parallel to the equator. But because this motion hath, by reason of its greater slowness, less force to thrust off the air in the parallel circles than in the equator, and no force at all at the poles, it may well be thought (for it is a certain consequent) that heavy bodies descend with less and less velocity, as they are more and more remote from the equator; and that at the poles themselves, they will either not descend at all, or not descend by the axis; which whether it be true or false, experience must determine. But it is hard to make the experiment, both because the times of their descents cannot be easily measured with sufficient exactness, and also because the places near the poles are inaccessible. Nevertheless, this we know, that by how much the nearer we come to the poles, by so much the greater are the flakes of the snow that falls; and by how much the more swiftly such bodies descend as are fluid and dissipable, by so much the smaller are the particles into which they are dissipated.

5. Supposing, therefore, this to be the cause of the descent of heavy bodies, it will follow that their motion will be accelerated in such manner, as that the spaces, which are transmitted by them in the several times, will have to one another the same proportion which the odd numbers have in succession from unity. For if the strait line E A be divided into any number of equal parts, the heavy body descending will, by reason of the perpetual action of the diurnal motion, receive from the air in every one of those times, in every several point of the strait line E A, a several new and equal impulsion; and therefore also in every one of those times, it will acquire a several and equal degree of celerity. And from hence it follows, by that which Galileus hath in his Dialogues of Motion demonstrated, that heavy bodies descend in the several times with such differences of transmitted spaces, as are equal to the differences of the square numbers that succeed one another from unity; which square numbers being 1, 4, 9, 16, &c. their differences are 3, 5, 7, &c.; that is to say, the odd numbers which succeed one another from unity. Against this cause of gravity which I have given, it will perhaps be objected, that if a heavy body be placed in the bottom of some hollow cylinder of iron or adamant, and the bottom be turned upwards, the body will descend, though the air above cannot depress it, much less accelerate its motion. But it is to be considered that there can be no cylinder or cavern, but such as is supported by the earth, and being so supported is, together with the earth, carried about by its diurnal motion. For by this means the bottom of the cylinder will be as the superficies of the earth; and by thrusting off the next and lowest air, will make the uppermost air depress the heavy body, which is at the top of the cylinder, in such manner as is above explicated.

6. The gravity of water being so great as by experience we find it is, the reason is demanded by many, why those that dive, how deep soever they go under water, do not at all feel the weight of the water which lies upon them. And the cause seems to be this, that all bodies by how much the heavier they are, by so much the greater is the endeavour by which they tend downwards. But the body of a man is heavier than so much water as is equal to it in magnitude, and therefore the endeavour downwards of a man's body is greater than that of water. And seeing all endeavour is motion, the body also of a man will be carried towards the bottom with greater velocity than so much water. Wherefore there is greater reaction from the bottom; and the endeavour upwards is equal to the endeavour downwards, whether the water be pressed by water, or by another body which is heavier than water. And therefore by these two opposite equal endeavours, the endeavour both ways in the water is taken away; and consequently, those that dive are not at all pressed by it.

7. If a body float upon the water, the weight of that body is equal to the weight of so much water as would fill the place which the immersed part of the body takes up within the water.

Let E F (in fig. 3) be a body floating in the water A B C D; and let the part E be above, and the other part F under the water. I say, the weight of the whole body E F is equal to the weight of so much water as the space F will receive. For seeing the weight of the body E F forceth the water out of the space F, and placeth it upon the superficies A B, where it presseth downwards; it follows, that from the resistance of the bottom there will also be an endeavour upwards. And seeing again, that by this endeavour of the water upwards, the body E F is lifted up, it follows, that if the endeavour of the body downwards be not equal to the endeavour of the water upwards, either the whole body E F will, by reason of that inequality of their endeavours or moments, be raised out of the water, or else it will descend to the bottom. But it is supposed to stand so, as neither to ascend nor descend. Wherefore there is an equilibrium between the two endeavours; that is to say, the weight of the body E F is equal to the weight of so much water as the space F will receive; which was to be proved.

8. From hence it follows, that any body, of how great magnitude soever, provided it consist of matter less heavy than water, may nevertheless float upon any quantity of water, how little soever.

Let A B C D (in fig. 4) be a vessel; and in it let E F G H be a body consisting of matter which is less heavy than water; and let the space A G C F be filled with water. I say, the body E F G H will not sink to the bottom D C. For seeing the matter of the body E F G H is less heavy than water, if the whole space without A B C D were fall of water, yet some part of the body E F G H, as E F I K, would be above the water; and the weight of so much water as would fill the space I G H K would be equal to the weight of the whole body E F G H; and consequently G H would not touch the bottom D C. As for the sides of the vessel, it is no matter whether they be hard or fluid; for they serve only to terminate the water; which may be done as well by water as by any other matter how hard soever; and the water without the vessel is terminated somewhere, so as that it can spread no farther. The part therefore E F I K will be extant above the water A G C F which is contained in the vessel. Wherefore the body E F G H will also float upon the water A G C F, how little soever that water be; which was to be demonstrated.

9. In the 4th article of chapter XXVI, there is brought for the proving of vacuum the experiment of water enclosed in a vessel; which water, the orifice above being opened, is ejected upwards by the impulsion of the air. It is therefore demanded, seeing water is heavier than air, how that can be done. Let the second figure of the same, chapter XXVI be considered, where the water is with great force injected by a syringe into the space F G B. In that injection, the air (but pure air) goeth with the same force out of the vessel through the injected water. But as for those small bodies, which formerly I supposed to be intermingled with air and to be moved with simple motion, they cannot, together with the pure air, penetrate the water; but remaining behind are necessarily thrust together into a narrower place, namely into the space which is above the water F G. The motions therefore of those small bodies will be less and less free, by how much the quantity of the injected water is greater and greater; so that by their motions falling upon one another, the same small bodies will mutually compress each other, and have a perpetual endeavour of regaining their liberty, and of depressing the water that hinders them. Wherefore, as soon as the orifice above is opened, the water which is next it will have an endeavour to ascend, and will therefore necessarily go out. But it cannot go out, unless at the same time there enter in as much air; and therefore both the water will go out, and the air enter in, till those small bodies which were left within the vessel have recovered their former liberty of motion; that is to say, till the vessel be again filled with air, and no water be left of sufficient height to stop the passage at B. Wherefore I have shown a possible cause of this phenomenon, namely, the same with that of thunder. For as in the generation of thunder, the small bodies enclosed within the clouds, by being too closely pent together, do by their motion break the clouds, and restore themselves to their natural liberty; so here also the small bodies enclosed within the space which is above the strait line F G, do by their own motion expel the water as soon as the passage is opened above. And if the passage be kept stopped, and these small bodies be more vehemently compressed by the perpetual forcing in of more water, they will at last break the vessel itself with great noise.

10. If air be blown into a hollow cylinder, or into a bladder, it will increase the weight of either of them a little, as many have found by experience, who with great accurateness have tried the same. And it is no wonder, seeing, as I have supposed, there are intermingled with the common air a great number of small hard bodies, which are heavier than the pure air. For, the ethereal substance, being on all sides equally agitated by the motion of the sun, hath an equal endeavour towards all the parts of the universe; and, therefore, it hath no gravity at all.

11. We find also by experience, that, by the force of air enclosed in a hollow cannon, a bullet of lead may with considerable violence be shot out of a gun of late invention, called the wind-gun. In the end of this cannon there are two holes, with their valves on the inside, to shut them close; one of them serving for the admission of air, and the other for the letting of it out. Also, to that end which serves for the receiving in of air, there is joined another cannon of the same metal and bigness, in which there is fitted a rammer which is perforated, and hath also a valve opening towards the former cannon. By the help of this valve the rammer is easily drawn back, and letteth in air from without; and being often drawn back and returned again with violent strokes, it forceth some part of that air into the former cannon, so long, till at last the resistance of the enclosed air is greater than the force of the stroke. And by this means men think there is now a greater quantity of air in the cannon than there was formerly, though it were full before. Also, the air thus forced in, how much soever it be, is hindered from getting out again by the aforesaid valves, which the very endeavour of the air to get out doth necessarily shut. Lastly, that valve being opened which was made for the letting out of the air, it presently breaketh out with violence, and driveth the bullet before it with great force and velocity.

12. Water, contrary to the custom of heavy bodies, ascendeth in the weather-glass; but it doth it when the air is cold: for when it is warm it descendeth again. And this organ is called a thermometer or thermoscope, because the degrees of heat and cold are measured and marked by it. It is made in this manner. Let A B C D (in fig. 5) be a vessel full of water, and E F G a hollow cylinder of glass, closed at E and open at G. Let it be heated, and set upright within the water to F; and let the open end reach to G. This being done, as the air by little and little grows colder, the water will ascend slowly within the cylinder from F towards E; till at last the external and internal air coming to be both of the same temper, it will neither ascend higher nor descend lower, till the temper of the air be changed. Suppose it, therefore, to be settled anywhere, as at H. If now the heat of the air be augmented, the water will descend below H; and if the heat be diminished, it will ascend above it. Which, though it be certainly known to be true by experience, the cause, nevertheless, hath not as yet been discovered.

In the sixth and seventh articles of chapter XXVIII, where I consider the cause of cold, I have shown, that fluid bodies are made colder by the pressure of the air, that is to say, by a constant wind that presseth them. For the same cause it is, that the superficies of the water is pressed at F; and having no place, to which it may retire from this pressure, besides the cavity of the cylinder between H and E, it is therefore necessarily forced thither by the cold, and consequently it ascendeth more or less, according as the cold is more or less increased. And again, as the heat is more intense or the cold more remiss, the same water will be depressed more or less by its own gravity, that is to say, by the cause of gravity above explicated.

13. Also living creatures, though they be heavy, can by leaping, swimming and flying, raise themselves to a certain degree of height. But they cannot do this except they be supported by some resisting body, as the earth, the water and the air. For these motions have their beginning from the contraction, by the help of the muscles, of the body animate. For to this contraction there succeedeth a distension of their whole bodies; by which distension, the earth, the water, or the air, which supporteth them, is pressed; and from hence, by the reaction of those pressed bodies, living creatures acquire an endeavour upwards, but such as by reason of the gravity of their bodies is presently lost again. By this endeavour, therefore, it is, that living creatures raise themselves up a little way by leaping, but to no great purpose: but by swimming and flying they raise themselves to a greater height; because, before the effect of their endeavour is quite extinguished by the gravity of their bodies, they can renew the same endeavour again.

14. The diaphanous medium, which surrounds the eye on all sides, is invisible; nor is air to be seen in air, nor water in water, nor anything but that which is more opacous. But in the confines of two diaphanous bodies, one of them may be distinguished from the other. It is not therefore a thing so very ridiculous for ordinary people to think all that space empty, in which we say is air; it being the work of reason to make us conceive that the air is anything. For by which of our senses is it, that we take notice of the air, seeing we neither see, nor hear, nor taste, nor smell, nor feel it to be anything? When we feel heat, we do not impute it to the air, but to the fire: nor do we say the air is cold, but we ourselves are cold; and when we feel the wind, we rather think something is coming, than that any thing is already come. Also, we do not at all feel the weight of water in water, much less of air in air. That we come to know that to be a body, which we call air, it is by reasoning; but it is from one reason only, namely, because it is impossible for remote bodies to work upon our organs of sense but by the help of bodies intermediate, without which we could have no sense of them, till they come to be contiguous. Wherefore, from the senses alone, without reasoning from effects, we cannot have sufficient evidence of the nature of bodies.

For there is underground, in some mines of coals, a certain matter of a middle nature between water and air, which nevertheless cannot by sense be distinguished from air; for it is as diaphanous as the purest air; and, as far as sense can judge, equally penetrable. But if we look upon the effect, it is like that of water. For when that matter breaks out of the earth into one of those pits, it fills the same either totally or to some degree; and if a man or fire be then let down in it, it extinguishes them in almost as little time as water would do. But for the better understanding of this phenomenon, I shall describe the 6th figure. In which let A B represent the pit of the mine; and let part thereof, namely C B, be supposed to be filled with that matter. If now a lighted candle be let down into it below C, it will as suddenly be extinguished as if it were thrust into water. Also, if a grate filled with coals thoroughly kindled and burning never so brightly, be let down, as soon as ever it is below C, the fire will begin to grow pale, and shortly after, losing its light, be extinguished, no otherwise than if it were quenched in water. But if the grate be drawn up again presently, whilst the coals are still very hot, the fire will, by little and little, be kindled again, and shine as before. There is, indeed, between this matter and water this considerable difference, that it neither wetteth, nor sticketh to such things as are put down into it, as water doth; which, by the moisture it leaveth, hindereth the kindling again of the matter once extinguished. In like manner, if a man be let down below C, he will presently fall into a great difficulty of breathing, and immediately after into a swoon, and die unless he be suddenly drawn up again. They, therefore, that go down into these pits, have this custom, that as soon as ever they feel themselves sick, they shake the rope by which they were let down, to signify they are not well, and to the end that they may speedily be pulled up again. For if a man be drawn out too late, void of sense and motion, they dig up a turf, and put his face and mouth into the fresh earth; by which means, unless he be quite dead, he comes to himself again, by little and little, and recovers life by breathing out, as it were, of that suffocating matter, which he had sucked in whilst he was in the pit; almost in the same manner as they that are drowned come to themselves again by vomiting up the water. But this doth not happen in all mines, but in some only; and in those not always, but often. In such pits as are subject to it, they use this remedy. They dig another pit, as D E, close by it, of equal depth, and joining them both together with one common channel, E B, they make a fire in the bottom E, which carries out at D the air contained in the pit D E; and this draws with it the air contained in the channel E B; which, in like manner, is followed by the noxious matter contained in C B; and, by this means, the pit is for that time made healthful. Out of this history, which I write only to such as have had experience of the truth of it, without any design to support my philosophy with stories of doubtful credit, may be collected the following possible cause of this phenomenon; namely, that there is a certain matter fluid and most transparent, and not much lighter than water, which, breaking out of the earth, fills the pit to C; and that in this matter, as in water, both fire and living creatures are extinguished.

15. About the nature of heavy bodies, the greatest difficulty ariseth from the contemplation of those things which make other heavy bodies ascend to them; such as jet, amber, and the loadstone. But that which troubles men most is the loadstone, which is also called Lapis Herculeus; a stone, though otherwise despicable, yet of so great power that it taketh up iron from the earth, and holds it suspended in the air, as Hercules did Antæus. Nevertheless, we wonder at it somewhat the less, because we see jet draw up straws, which are heavy bodies, though not so heavy as iron. But as for jet, it must first be excited by rubbing, that is to say, by motion to and fro; whereas the loadstone hath sufficient excitation from its own nature, that is to say, from some internal principle of motion peculiar to itself. Now, whatsoever is moved, is moved by some contiguous and moved body, as hath been formerly demonstrated. And from hence it follows evidently, that the first endeavour, which iron hath towards the loadstone, is caused by the motion of that air which is contiguous to the iron: also, that this motion is generated by the motion of the next air, and so on successively, till by this succession we find that the motion of all the intermediate air taketh its beginning from some motion which is in the loadstone itself; which motion, because the loadstone seems to be at rest, is invisible. It is therefore certain, that the attractive power of the loadstone is nothing else but some motion of the smallest particles thereof. Supposing, therefore, that those small bodies, of which the loadstone is in the bowels of the earth composed, have by nature such motion or endeavour as was above attributed to jet, namely, a reciprocal motion in a line too short to be seen, both those stones will have one and the same cause of attraction. Now in what manner and in what order of working this cause produceth the effect of attraction, is the thing to be enquired. And first we know, that when the string of a lute or viol is stricken, the vibration, that is, the reciprocal motion of that string in the same strait line, causeth like vibration in another string which hath like tension. We know also, that the dregs or small sands, which sink to the bottom of a vessel, will be raised up from the bottom by any strong and reciprocal agitation of the water, stirred with the hand or with a staff. Why, therefore, should not reciprocal motion of the parts of the loadstone contribute as much towards the moving of iron? For, if in the loadstone there be supposed such reciprocal motion, or motion of the parts forwards and backwards, it will follow that the like motion will be propagated by the air to the iron, and consequently that there will be in all the parts of the iron the same reciprocations or motions forwards and backwards. And from hence also it will follow, that the intermediate air between the stone and the iron will, by little and little, be thrust away; and the air being thrust away, the bodies of the loadstone and the iron will necessarily come together. The possible cause therefore why the loadstone and jet draw to them, the one iron, the other straws, may be this, that those attracting bodies have reciprocal motion either in a strait line, or in an elliptical line, when there is nothing in the nature of the attracted bodies which is repugnant to such a motion.