On this model flutes, whistles, and other wind instruments are made.
4. It has been discovered that the substance that causes thunder in the clouds, and hurls down fire and stones, is saltpetre ignited in combination with sulphur. In imitation of this, gunpowder is now made out of sulphur and saltpetre. When this is ignited and fired from cannon, a mimic storm, with thunder and lightning, is produced.
5. It has been found that water always tends to preserve a level surface, even in vessels that communicate but are at some distance from one another. The experiment has been made of conducting water through pipes, and it has been found that it will rise from any depth to any height, provided that it originally fall from that height. This is an artificial arrangement, but it is also natural; for the exact mode in which the action takes place is artificial, but the law on which the action depends is natural.
6. The vault of heaven, on observation, has been found to revolve continuously, thus, by the various revolutions of the planets, producing the changes of the seasons which are so pleasant. In imitation of this an instrument has been devised, representing the daily revolution of the vault of heaven. It is composed of wheels arranged so that not only can one be driven by the other, but that all can be put into continuous motion. Now it was necessary to construct this instrument out of movable and immovable parts, as the universe itself is constructed, and consequently we find a solid pedestal, pillars, and circular rings, corresponding to the earth, the immovable element in the universe, while in place of the movable orbits in the heaven we have the various wheels. But because it was impossible to command any one wheel to turn round and to carry others with them (as the Creator gave the heavenly lights the power to move themselves, and others with them), the motive power has to be borrowed from nature, and a weight or a spring is used.
