violently kindled and hurled to the ground with great force as the clouds grind against each other or are torn by the wind. Others say that it is a compression of fiery air descending with great force. A typhoon is a great and violent thunderstorm whirlwind-like, or a whirlwind of smoke from a cloud that has burst. A " prester is a cloud rent all round by the force of fire and wind. Earthquakes, say they, happen when the wind finds its way into, or is imprisoned in, the hollow parts of the earth : so Posidonius in his eighth book ; and some of them are tremblings, others openings of the earth, others again lateral displacements, and yet others vertical displacements. They maintain that the parts of the world are arranged thus. The earth is in the middle answering to a centre ; next comes the water, which is shaped like a sphere all round it, concentric with the earth, so that the earth is in water. After the water comes a spherical layer of air. There are five celestial circles : first, the arctic circle, which is always visible ; second, the summer tropic ; third, the circle of the equinox ; fourth, the winter tropic ; and fifth, the antarctic, which is invisible to us. They are called parallel, because they do not incline towards one another ; yet they are described round the same centre. b The zodiac is an oblique circle, as it crosses the parallel circles. And there are five terrestrial zones : first, the northern zone which is beyond the arctic circle, uninhabitable because of the cold ; second, a temperate zone ; a third, uninhabitable because of great heats, called the torrid zone ; fourth, a counter-temperate zone ; fifth, the southern zone, uninhabitable because of its cold.