[On the propulsive force of rockets] One part of fire takes up as much space as ten parts of air, and one part of air takes up the space of ten parts of water, and one part of water as much as ten parts of earth. Now powder is earth, consisting of the four elementary principles, and when the sulfur conducts the fire into the dryest part of the powder, fire, and air increase ... the other elements also gird themselves for battle with each other and the rage of battle is changed by their heat and moisture into a strong wind.
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