68 Dr. Mac Culloch on the Granite Tors of Cornwall. 



the precipice. The breadth of the apparent contact betv/een tr\c 

 plane and the centre of motion of the stone is about a foot and a 

 half. As this support is curved only in one direction, being of a 

 cylindrical and not of a spheroidal figure, the motion of the stone 

 is consequently limited to a vibration in one direction, which is 

 nearly at right angles to its longest dimensions. The general 

 aspect of the stone would scarcely enable a cursory spectator to 

 assign the reason of its vibratory power, as from the point of view 

 in which it is usually seen, the centre of gravity would appear 

 placed rather above than below the centre of suspension. It is said 

 that the motion is now much more limited than it has been vv^ithin 

 the memory of those who live near it ; a circumstance rendered 

 very probable by the progress of disintegration at those points of 

 contact where water can be detained. A continuance indeed of the 

 very process to which it owes this property, must ultimately destroy 

 its motion if it operates by bringing a wider surface into contact, 

 thus defeating the enlarged vibration which would otherwise follow 

 from the increasing distance between its centres of suspension and 

 of gravity. A quantity of loose quartz gravel may be generally 

 found near the points of contact, marking the progress of this 

 disintegration. 



In the trials which I have at different times made on it, the 

 greatest force that three persons could apply to it was sufficient to 

 make its outer edge describe an arc whose chord was -f of an inch 

 at 6 feet distance from the centre of motion. When suffered to 

 return it vibrates for a few seconds before it falls again to rest. A 

 force of a very few pounds is however sufficient to bring it into a 

 state of vibration, and to maintain a visible motion. Even the 

 wind blowing on its western exposed surface produces this effect in 

 a very sensible degree. It is the largest of its kind at present 



