assumed to accompany Earthquakes. 51<1 



The impelling force is the grasp of the stone, which its bed 

 holds of it by friction or adhesion ; and this may also be re- 

 ferred to some one point in the surfaces of contact, which we 

 might call the centre of adherence. 



If, then, a stone or" other solid body rest upon a horizontal 

 plane, which is suddenly moved with sufficient velocity to 

 effect motion in the incumbent body, three several conditions 

 of motion of the body may occur, according to the respective 

 position of the centre of gravity of the stone, and of the centre 

 of adherence. 



1st. The centre of gravity of the stone may be at such a 

 height above the base, that it shall upset by its own inertia. 

 This is the case with houses, towers, walls, &c, when they 

 fall by earthquakes, accompanied also by dislocation of their 

 parts. 



2nd. The centre of adherence may be in a point of the 

 base, plumb under the centre of gravity of the stone ; or in a 

 vertical plane, passing through the centre of gravity of the 

 stone, and in the direction of motion of the base. 



In this case, the stone will appear to move in the opposite 

 direction to that in which the base has moved ; that is to say, 

 the stone may have acquired more or less the direction of mo- 

 tion of the base, according as the motion of the latter has been 

 longer or shorter continued, or less or more rapid ; but, in so 

 far as the movement in the opposite direction has taken place, 

 the base, in reality, has slipped from under the stone. 



3rd. The centre of adherence may neither be plumb under 

 the centre of gravity of the stone, nor in the plane of motion 

 passing through its centre of gravity, but in some point of the 

 base outside the line of its intersection by this plane; in which 

 case, the effect of the horizontal rectilineal motion of the base 

 will be to twist the stone round upon its bed, or to move it 

 laterally, and twist it at the same time, thus converting the 

 rectilineal into a curvilineal motion, in space ; the relative 

 amount of the two compounded motions being dependent upon 

 the velocity and time of movement of the base, and upon the 

 perpendicular distance measured horizontally at the surface of 

 adherence, between the centre of adherence and the centre of 

 gravity of the stone. 



This latter case is that which applies to the twisted stones 

 of Calabria, South America and Greece ; and affords, as I 

 feel assured, the true explanation of the phaenomena. 



The relation of these forces, which have taken so many 

 words to state correctly, might, of course, have been expressed 

 algebraically in three lines; but as this would not be uni- 

 versally intelligible, I have preferred the more tedious and in- 



