252 PHYSICAL SCIENCE BK. vi 



if you withdraw wind, water is sluggish. Water 

 becomes impetuous only when the blast tosses it 

 with violence. This force it is that has power to 

 scatter vast spaces of earth, to raise from the 

 depths new mountains, and to set in mid -ocean 



2 islands hitherto unseen. Can any one doubt that 

 There and Therasia and this island which in our 

 days under our very eyes rose out of the Aegean 

 Sea, were carried up to the light by the force of 

 air ? 



Posidonius will have it that there are two 

 different varieties in the movements of the earth, 

 each with its distinctive name. The one is a 

 quaking when the earth is shaken and moves up 

 and down ; the other is a tilting when, like a 



3 ship, it leans over to one or other side. I am of 

 opinion that there is still a third variety, which we 

 have a special term to denote. Our forefathers had 

 good reason for speaking of a trembling of the 

 earth, for it is unlike either of the other kinds of 

 movement. On such an occasion things are neither 

 all shaken nor all tilted, but they quiver. In a case 

 of this kind no great damage is usually done ; while, 

 on the other hand, a tilting is far more destructive 

 than a shock ; for unless a contrary movement set 

 in very quickly from the other side to restore the 

 level, downfall follows of necessity. 



XXII 



THESE movements being dissimilar, their causes are 

 likewise different. Let us deal first with the shaking 

 movement. If great loads are being conveyed by 

 a row of many waggons, and the wheels, under 



