Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/78

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Dr. MacCulloch on the Granite Tors of Cornwall.

the precipice. The breadth of the apparent contact between the 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 within 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 3/4 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