XVIII
1
The chief cause of earthquake, therefore, is air, an element naturally swift and shifting from place to place. As long as it is not stirred, but lurks in a vacant space, it reposes innocently, giving no trouble to objects round it. But when any cause coming upon it from without rouses it, or compresses it, and drives it into a narrow space, in the first instance, to be sure, it merely retires and roams about its enclosure. But when opportunity of escape is cut off, and resistance meets it on all hands, then
... With deep murmur of the mountain
It roars around the barriers;...
2
3
which, after long battering, it dislodges and tosses on high, growing the more fierce, the stronger the obstacle with which it has contended. By and by, when it has traversed the whole space in which it was enclosed, and has failed to find a way of escape, it recoils from the side on which its impact was greatest. It is then either distributed through the secret openings which the earthquake of itself causes here and there, or escapes through a new rent. So uncontrollable is this mighty power. No bolt can imprison wind; it loosens every bond, bears with it every weight, and insinuating itself into the smallest crannies wins its release; for by the invincible power of nature it is free, especially when roused, and asserts its right for itself. Air is a thing no man can tame; nothing will be found which,
When the winds struggle and the tempests roar,
Can restrain them by its sway and rein them by bonds and prison.
4
Doubtless the poets wished the place in which the winds lay pent up underground to be considered a prison. But they did not perceive either that what was shut up is no longer wind, or that what is wind can no longer be shut up. What is shut up is at rest, and the atmosphere is at a standstill; whereas all wind is in flight. Besides these arguments, there is a consideration by which it becomes manifest that motion is brought about by air, namely, that our bodies never tremble except when some cause produces disturbance of the internal air,84 which is contracted by fear, grows sluggish in old age, languishes when the veins are numbed, is checked with cold, or after some attack of fever is quite driven from its wonted course. As long as it flows unimpeded, and moves in its wonted fashion, there is no quivering of the body. When anything intervenes to prevent its functioning, then being no longer able to maintain what it upheld by its vigour, it fails, causing a collapse of everything that it had sustained when unimpaired.