slight and constantly varying elements of argument. One other and still more remarkable, class of works remains to be noted: These are the Rocking Stones, which are found among the ancient monuments of England and Ireland, as well as on various parts of the Continent, and are no less frequent in Scotland. No evidences of ancient skill or of primitive superstitious rites are more calculated to awaken our astonishment and admiration of their singular constructors. There is so strange a mixture of extreme rudeness and great mechanical skill in these memorials of the remote past, that they excite greater wonder and awe in the thoughtful mind than even the imposing masses inclosing the sacred area of Stonehenge or the circle of Stennis. It would, I imagine, prove a much more complicated problem for the modern engineer to poise the irregular and amorphous mass on its point of equilibrium, than to rear the largest monolithic group that now stands to attest the mechanical power which the old builders could command. It has indeed been supposed by some that the origin of Rocking Stones is traceable entirely to natural causes, and this opinion is now adopted by Worsaae and other Danish and Norwegian antiquaries.[147] Such a theory, however, seems to stand fully as much in need of proof as that which regards them as stones of ordeal, by which the Druid or Scandinavian priests were wont to test the guilt or innocence of the accused. Apollonius Rhodius speaks of rocking stones placed on the apex of tumuli, and Mr. Akerman refers, in his Archæological Index, to the famous Agglestone Barrow, in the island of Purbeck, as having been similarly surmounted.[148] One such undoubted example would abundantly suffice to overthrow this geological theory of natural formation. It is a less conclusive, though not altogether valueless argument, that some of the most remarkable logan stones of Scotland are found in the immediate vicinity of other undoubted primitive stone-works. The great rocking stone in the parish of Kirkmichael, Perthshire, for example, has already been referred to as one of a large group of stone circles, cairns, and other monuments of the same class. Its form is that of a rhombus, of which the greater diagonal is seven feet, and the less five feet, and its weight is calculated at about three tons and half a hundredweight. On pressing down either of the extreme corners, a rocking motion is produced, which increases until the arc through which its longest radius moves exceeds a foot. When the pressure has been continued so as to produce this effect, the stone makes from twenty-six to twenty-eight vibrations from side to side after it is withdrawn. A much larger rocking stone is situated on the Hill of Mealyea, in the parish of Kells, Stewartry of Kirkcudbright. Its weight is estimated at from eight to ten tons, and it is so nicely poised that it can be set in motion with the pressure of the finger. To this the name