The sun draws from the sea and from rivers and other wet places, vapours and mists which, owing to their thinness, are invisible except in the morning and evening, and when they are drawn up into the hot air, they are scattered and spread and mingled with the air, since they are of the same nature. On the other hand, when they are drawn up into the cold air, they become compressed and contracted within themselves and they are converted into clouds, and since it is the nature of like things to approach each other, as the rivers enter the sea, so do the lesser of these clouds approach the larger clouds since they are lighter and can move more readily. And they become one large dark mass, and since that mass is warm by nature, and the cold air surrounds it, they are opposed, and contend with each other.
When the air is the stronger, and overcomes the cloud, it binds and condenses the edges without, and converts it into snow. Consequently, when the heat is inside in the cloud, and it is surrounded by the cold without, with the cloud freezing and hardening around it, it would seek, according to its nature, a place where it could extend and dilate and spread; and since the dense cloud does not suffer it to do so, the heat shakes it powerfully, and it the cloud breaks, and a great and terrible sound, called thunder, results from that breaking, and with the strength of the force by which that rupture is caused, thunder-bolts and lightning result from that rupture, and small fragments of that cloud fall, striking and breaking against each other. As they descend, they break each other again into small pieces, and when they come in contact
p.153with the part of the atmosphere nearest the earth the heat of that place deprives them of their knobby points, and renders them spherical, and the hailstones fall, and the small drops of rain that mingle with the snow come from the part which it loses as it melts.
The greater the aforementioned heat and cold, the greater the opposition between them, and as the opposition is increased, the thunder and lightning which results from them is increased. The part of the cloud which does not fall to the earth spreads throughout the atmosphere, and is converted into lightning. The part of the lightning which comes to earth splits hills and mountains, and penetrating the earth, kills men and cattle.
As a proof that thunder results from the contrariety I mentioned, the philosophers have cited an example: when a green leaf is put upon fire, before it burns, when the heat comes in contact with it, it breaks with a sound. In the same way when red hot iron is put into water, the contrariety of these two things draws a tremendous noise from them. Then since the contrariety of small bodies produces this noise, large bodies ought to produce a great noise.
There are more thunder, lightning and thunderbolts in spring and autumn, than in the other seasons, because these two seasons occur between the warm summer and the cold winter. The clouds which the blowing of the wind draws up from the earth into the cold, wet, thin attenuated air, without heat or dryness, except what is contained in the clouds themselves, possess no contrariety.
The heavy part which is contained in those clouds separates from them in drops, and is converted into rain, and when the cloud meets the warm air, it the air rarefies it and converts it back into air, and through the disagreement due to the contrariety of the heat and cold, dryness and wetness of that air, it is changed into large black clouds, and those black clouds are changed into heavy rain; and sometimes the same substance is converted into large drops of rain and great hailstones, which occur most frequently in spring and autumn, and when they occur in the summer,
p.155on account of that season possessing so much of the contrariety I mentioned, compared to the other seasons, the tempest is greater then. When a great wind accompanies that tempest, it gathers the clouds together up in the sky, and binds them, and makes them assume different shapes, and ill-informed people think that they are dragons. We perceive the dust of the earth being whirled around by the wind in the same way.
Although the thunder and lightning are produced simultaneously, the lightning is seen before the thunder is heard. The reason of that is that the eye sees what is near it and what is distant from it in the same way, for it does not perceive the earth any sooner than it does the stars that are most distant from it in the firmament. That is not the case with the hearing, for one hears the sound that is near sooner than the sound which is distant; and in explaining that, the doctors compared the sense of hearing to a quern, for if there were an ear in the opening of the quern, it would hear everything near to it and distant from it indiscriminately, because the sense of the ear is like air, which is a thin, rarefied movable body, the motion of which is greater, smoother, and swifter than that of water.
When some disruption, or striking, or other noise occurs in the air, the air which is nearest that noise propels the sound away from it, towards the other parts of the air, until finally it enters the ear, and passes from the ear to the brain, which distinguishes between the greatest and the least, and between the gentlest and the loudest noise.
In the same way, they compared the sense of sight to a trumpet which has a narrow end, and the further from the end it is the broader it becomes, and thus the sight of the eye passes through the sinewy vein from the brain
p.157to the pupils of the eyes, and there has a narrow end like a trumpet, and it widens out until it meets the object which it beholds, and turns in again, carrying the shape form, and colour of that object with it to the brain.