On the friction of the heavens: whether it makes sound or not. Every sound is caused by air struck against a dense body, and if it is made by two heavy bodies against each other, it is by means of the air that surrounds them, and this friction consumes the bodies rubbed together. It would therefore follow that the heavens, in their friction, having no air between them, would generate no sound; and if such friction did indeed have reality, in the many centuries that these heavens have revolved they would have been consumed by such immense velocity performed every day. And even if it did make sound, that sound could not spread, because the sound of a blow struck under water is little heard, and less or nothing would be heard in dense bodies. Again, in polished bodies their friction makes no sound, which likewise would happen — that no sound is made — in the contact or friction of the heavens. And if these heavens are not polished in the contact of their frictions, it follows that they are globular and rough. Therefore their contact is not continuous. This being so, a vacuum is generated, which is concluded not to occur in nature. Therefore it is concluded that friction would have consumed the boundaries of each heaven, and all the more the faster it is toward the middle than toward the poles, so that it would wear away more in the middle than at the poles, and then it would rub no more, and the sound would cease, and the dancers would stop — unless the heavens, one turned to the east and the other to the north.