540 Mr. R. Mallet on the Vorticose Movement, 



should be so great as to wrench out of its mortared bed, and 

 twist a block of masonry by merely its own inertia. 



Considering these circumstances, on lately reading the fore- 

 going passages of Darwin, I was soon led to see that the twist- 

 ing phaenomena observed could be readily accounted for upon 

 the established principles of mechanics, without having re- 

 course to either vortices or vibrations, arranging blocks of 

 many hundred weights, after the manner of pins on paper, or 

 sand on one of Chladni's acoustic plates, — an explanation 

 which, with all my admiration of Darwin, appears quite as far 

 from probability as its predecessor. 



I assume, then, nothing more than what is universally ad- 

 mitted, that during earthquakes a motion of some sort takes 

 place, by which the ground itself, and all objects resting upon 

 it, are shaken or moved back and forwards, by an alternate 

 horizontal motion, within certain narrow limits, which, for all 

 present evidence to the contrary, may be a straight line mo- 

 tion, though possibly variable in direction at different, and 

 sometimes closely successive times, and the velocity of which 

 is sufficient to throw down or disturb the position of bodies 

 supported by the earth, through their own inertia. 



Let us now apply this to the cases described of stones 

 twisted on their bases, and the explanation will at once come 

 to light. 



If a stone, whether symmetrical or otherwise, rest upon a 

 given base, and that motion be suddenly communicated hori- 

 zontally to that base in any direction, the stone itself will be 

 solicited to move in the same direction, and the measure of 

 force with which the movement of the base is capable of affect- 

 ing the stone or other incumbent body, is equal to the amount 

 of friction of the latter upon its base — a function of its weight 

 which, without the intervention of cement, may be from one- 

 fifth to one-tenth of the weight of the body, for cut stone rest- 

 ing on cut stone, but may be increased to any amount by the 

 intervention of cement. 



The stone, however, is possessed of weight, and therefore 

 of inertia ; that is to say, being at rest, its whole mass cannot 

 be instantly brought into motion by the plane, and if the 

 amount of adhesion between the stone and its bed be less than 

 the inertia due to any given velocity of horizontal movement 

 of the bed, the bed will move more or less from under the 

 stone, or the stone will appear to move in a contrary direction 

 to that of the motion of its bed. 



Now the inertia of the stone, which is here the resisting 

 force, may be considered to act at the centre of gravity of the 

 body. 



