Why a bladder is heavier when blown full of air, than when it is empty.
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.
The cause of the ejection upwards of heavy bodies from a wind-gun.
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.
As for the cause of this, I could easily attribute it, as most men do, to condensation, and think that the air, which had at the first but its ordinary degree of rarity, was afterwards, by the forcing in of more air, condensed, and last of all, rarified again by being let out and restored to its natural liberty. But I cannot imagine how the same place can be always full, and, nevertheless, contain some times a greater, sometimes a less quantity of matter; that is to say, that it can be fuller than full. Nor can I conceive how fulness can of itself be an efficient cause of motion. For both these are impossible. Wherefore we must seek out some other possible cause of this phenomenon. Whilst, therefore, the valve which serves for the letting in of air, is opened by the first stroke of the rammer, the air within doth with equal force resist the entering of the air from without; so that the endeavours between the internal and external air are opposite, that is, there are two opposite motions whilst the one goeth in and the other cometh out; but no augmentation at all of air within the cannon. For there is driven out by the stroke as much pure air, which passeth between the rammer and the sides of the cannon, as there is forced in of air impure by the same stroke. And thus, by many forcible strokes, the quantity of small hard bodies will be increased within the cannon, and their motions also will grow stronger and stronger, as long as the matter of the cannon is able to endure their force; by which, if it be not broken, it will at least be urged every way by their endeavour to free themselves; and as soon as the valve, which serves to let them out, is opened, they will fly out with violent motion, and carry with them the bullet which is in their way. Wherefore, I have given a possible cause of this phenomenon.
The cause of the ascent of water in a weather-glass.
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.
Cause of motion upwards in living creatures
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.