XXI. Air must be the cause. Different kinds of earthquakes 251 XXII. First species--shaking of the earth: its causes 252 XXIII. Next comes the form of concussion caused by air. The great Callisthenes, who braved the fury of Alexander and lost his life for it, supports this view. Submarine effects of it are particularly noticeable 253 XXIV. Different explanations may be given of the exact method in which air acts 255 XXV. The striving of the air in subterranean caverns produces a concussion or collapse in the earth above. The area of disturbance is limited, never over 200 miles, as numerous instances prove. The Peneüs and Ladon were thus produced 256 XXVI. The nature of the soil composed of muddy accretions without interstices is said to account for the exemption of Egypt from earthquakes. So Delos in the sea has porous rocks which emit the air easily. But the facts are wrong. There is abundant proof that proximity to the sea is no safeguard against shock 258 XXVII. A peculiarity of the Campanian earthquake, that it killed 600 sheep, is explained by the emission of pestilential vapour, by which sheep, with their heads close to the ground, naturally were most readily affected 259 XXVIII. Noxious vapours are not, however, peculiar to earthquakes. They are found in several parts of Italy habitually. Such, too, is the origin of new diseases 261 XXIX. Excessive fear drives people mad. Earthquakes split statues and divide kingdoms, _e.g._ Sicily from Italy, Spain from Africa 262 XXX. The action of the air accounts for all the detailed phenomena, splitting of walls, houses, towers, statues; also for the prolongation of shocks for several days 263 XXXI. A further proof that air is the agent is to be found in the gradually diminishing violence of the successive shocks. Phenomena in the pavement witnessed by a philosopher who was in his bath 264 XXXII. The moral. Life hangs on a thread; why should one dread the loss of it? The greatness of the cause of death is no source of terror. The hereafter is better and safer than earth. There there is no fear of earthquake or thunderstorm, fire or flood. Fear of death magnifies all human risks. Do not dread death, long for it, and, if necessary, meet it half way 265 BOOK VII TREATING OF COMETS