“In the parish of S. Levan, there is a promontory called Castle Treryn. This cape consists of three distinct groups of rocks. On the western side of the middle group near the top, lies a very large stone, so evenly, poised that any hand may move it to and fro; but ......... it is morally impossible that any lever, or indeed force, however applied in a mechanical way, can remove it from its present situation.”