5. We perceive a body to be hard, from this, that, when touching it, we would thrust forwards that part of the same which we touch, we cannot do it otherwise than by thrusting forwards the whole body. We may indeed easily and sensibly thrust forwards any particle of the air or water which we touch, whilst yet the rest of its parts remain to sense unmoved. But we cannot do so to any part of a stone. Wherefore I define a hard body to be that whereof no part can be sensibly moved, unless the whole be moved. Whatsoever therefore is soft or fluid, the same can never be made hard but by such motion as makes many of the parts together stop the motion of some one part, by resisting the same.
What it the cause of cold near the poles.
6. Those things premised, I shall show a possible cause why there is greater cold near the poles of the earth, than further from them. The motion of the sun between the tropics, driving the air towards that part of the earth's superficies which is perpendicularly under it, makes it spread itself every way; and the velocity of this expansion of the air grows greater and greater, as the superficies of the earth comes to be more and more straitened, that is to say, as the circles which are parallel to the equator come to be less and less. Wherefore this expansive motion of the air drives before it the parts of the air, which are in its way, continually towards the poles more and more strongly, as its force comes to be more and more united, that is to say, as the circles which are parallel to the equator are less and less; that is, so much the more, by how much they are nearer to the poles of the earth. In those places, therefore, which are nearer to the poles, there is greater cold than in those which are more remote from them. Now this expansion of the air upon the superficies of the earth, from east to west, doth, by reason of the sun's perpetual accession to the places which are successively under it, make it cold at the time of the sun's rising and setting; but as the sun comes to be continually more and more perpendicular to those cooled places, so by the heat, which is generated by the supervening simple motion of the sun, that cold is again remitted; and can never be great, because the action by which it was generated is not permanent. Wherefore I have rendered a possible cause of cold in those places that are near the poles, or where the obliquity of the sun is great.
The cause of ice; and why the cold is more remiss in rainy than in clear weather. Why water doth not freeze in deep wells, as it doth near the superficies of the earth. Why ice is not so heavy as water; and why wine is not so easily frozen as water.
7. How water may be congealed by cold, may be explained in this manner. Let A (in [figure 1]) represent the sun, and B the earth. A will therefore be much greater than B. Let E F be in the plane of the equinoctial; to which let G H, I K, and L C be parallel. Lastly, let C and D be the poles of the earth. The air, therefore, by its action in those parallels, will rake the superficies of the earth; and that with motion so much the stronger, by how much the parallel circles towards the poles grow less and less. From whence must arise a wind, which will force together the uppermost parts of the water, and withal raise them a little, weakening their endeavour towards the centre of the earth. And from their endeavour towards the centre of the earth, joined with the endeavour of the said wind, the uppermost parts of the water will be pressed together and coagulated, that is to say, the top of the water will be skinned over and hardened. And so again, the water next the top will be hardened in the same manner, till at length the ice be thick. And this ice, being now compacted of little hard bodies, must also contain many particles of air received into it.
As rivers and seas, so also in the same manner may the clouds be frozen. For when, by the ascending and descending of several clouds at the same time, the air intercepted between them is by compression forced out, it rakes, and by little and little hardens them. And though those small drops, which usually make clouds, be not yet united into greater bodies, yet the same wind will be made; and by it, as water is congealed into ice, so will vapours in the same manner be congealed into snow. From the same cause it is that ice may be made by art, and that not far from the fire. For it is done by the mingling of snow and salt together, and by burying in it a small vessel full of water. Now while the snow and salt, which have in them a great deal of air, are melting, the air, which is pressed out every way in wind, rakes the sides of the vessel; and as the wind by its motion rakes the vessel, so the vessel by the same motion and action congeals the water within it.
We find by experience, that cold is always more remiss in places where it rains, or where the weather is cloudy, things being alike in all other respects, than where the air is clear. And this agreeth very well with what I have said before. For in clear weather, the course of the wind which, as I said even now, rakes the superficies of the earth, as it is free from all interruption, so also it is very strong. But when small drops of water are either rising or falling, that wind is repelled, broken, and dissipated by them; and the less the wind is, the less is the cold.
We find also by experience, that in deep wells the water freezeth not so much as it doth upon the superficies of the earth. For the wind, by which ice is made, entering into the earth by reason of the laxity of its parts, more or less, loseth some of its force, though not much. So that if the well be not deep, it will freeze; whereas if it be so deep, as that the wind which causeth cold cannot reach it, it will not freeze.
We find moreover by experience, that ice is lighter than water. The cause whereof is manifest from that which I have already shown, namely, that air is received in and mingled with the particles of the water whilst it is congealing.
Lastly, wine is not so easily congealed as water, because in wine there are particles, which, being not fluid, are moved very swiftly, and by their motion congelation is retarded. But if the cold prevail against this motion, then the outermost parts of the wine will be first frozen, and afterwards the inner parts; whereof this is a sign, that the wine which remains unfrozen in the midst will be very strong.