Question VII.

In what Sense Does Plato Say, that the Antiperistasis (Or Reaction) Of Motion - By Reason There is No Vacuum - Is the Cause Of the Phenomena in Physicians' Cupping-Glasses, in Swallowing, in Casting Weights, in the Running of Water, in Thunder, in The Attraction of the Loadstone, and in the Harmony of Sounds? (See "Timaeus," pp. 79-81.)

For it seems unreasonable to ascribe the reason of such different effects to the selfsame cause.

How respiration is made by the reaction of the air, he has sufficiently shown. But the others, he says, seem to be effected miraculously, but really the bodies force each other aside and change places with one another; while he has left for us to discover how each is particularly done.

As to cupping-glasses, the case is thus: the air next to the flesh being comprehended and inflamed by the heat, and being made more rare than the pores of the brass, does not go into a vacuum (for there is no such thing), but into the air that is without the cupping-glass, and has an impulse upon it. This air drives that before it; and each, as it gives way, strives to succeed into the place which was vacuated by the cession of the first. And so the air approaching the flesh comprehended by the cupping-glass, and attracting it, draws the humors into the cupping-glass.

Swallowing takes place in the same way. For the cavities about the mouth and stomach are full of air; when therefore the meat is squeezed down by the tongue and tonsils, the elided air follows what gives way, and also forces down the meat.

Weights also thrown cleave the air and dissipate it, as they fall with force; the air recoiling back, according to its proper tendency to rush in and fill the vacuum, follows the impulse, and accelerates the motion.

The fall also of thunderbolts is like to darting anything. For by the blow in the cloud, the fiery matter exploded breaks into the air; and it being broken gives way, and again being contracted above, by main force it presses the thunderbolt downwards contrary to Nature.

And neither amber nor the loadstone draws anything to it which is near, nor does anything spontaneously approach them. But this stone emits strong exhalations, by which the surrounding air being impelled forceth that which is before it; and this being drawn round in the circle, and returning into the vacuated place, forcibly draws the iron in the same movement. In amber there is a flammeous and spirituous nature, and this by rubbing on the surface is emitted by recluse passages, and does the same that the loadstone does. It also draws the lightest and driest of adjacent bodies, by reason of their tenuity and weakness; for it is not so strong nor so endued with weight and strength as to force much air and to act with violence and to have power over great bodies, as the magnet has. But what is the reason the air never draws a stone, nor wood, but iron only, to the loadstone? This is a common question both by those who think the coition of these bodies is made by the attraction of the loadstone, and by such as think it done by the incitement of the iron. Iron is neither so rare as wood, nor altogether so solid as gold or a stone; but has certain pores and asperities, which as far as inequality is concerned are proportionable to the air; and the air being received in certain positions, and having (as it were) certain stays to hang to, does not slip off; but when it is carried up to the stone and is forced against it, it draws the iron by force along with it to the stone. Such then may be the reason of this.

But the manner of the waters running over the earth is not so evident. But it is observable that the waters of lakes and ponds stand immovable, because the air about them stagnates immovable and admits of no vacuity. For the water on the surface of lakes and seas is troubled and fluctuates as the air is moved, it following the motion of the air, and moving as it is moved. For the force from below causes the hollowness of the wave, and from above the swelling thereof; until the air ambient and containing the water is still. Therefore the flux of such waters as follow the motion of the receding air, and are impelled by that which presses behind, is continued without end. And this is the reason that the stream increases with the waters, and is slow where the water is weak, the air not giving way, and therefore enduring less reaction. So the water of fountains must needs go upwards, the extrinsic air succeeding into the vacuity and throwing the water out. In a close house, that keeps in the air and wind, the floor sprinkled with water causes an air or wind, because, as the sprinkled water falls, the air gives way. For it is so provided by Nature that air and water force one another and give way to one another; because there is no vacuity in which one can be fixed without experiencing the change and alteration in the other.

Concerning symphony, he shows how sounds harmonize. A quick sound is acute, a slow is grave. Therefore acute sounds move the senses the quicker; and these dying and grave sounds supervening, what arises from the contemperation of one with the other causes pleasure to the ear, which we call harmony. And by what has been said, it may easily be understood that air is the instrument of these things. For sound is the stroke upon the sense of the hearer, caused by the air; and the air strikes as it is struck by the thing moving - if violent, acutely - if languid, softly. The violent stroke comes quick to the ear; then the circumambient air receiving a slower, it affects and carries the sense along with it.