evident that the circular fissure, as it descends into the body of the flint, will have a tendency to enlarge in diameter, so that the piece of flint it includes will be of conical form, the small circle struck by the hammer forming the slightly truncated apex. That this is not mere theory will be seen from the annexed woodcut, Fig. 188, showing a cone of flint produced by a single blow of a hammer.[1]
Fig. 188.—Artifical Cone of Flint.
Sometimes, as has been shown by Prof. T. McK. Hughes, F.R.S., the sides of the cone are in steps, the inclination varying from 30° to 110°. This is probably to some extent due to the character of the blow, and the form of the hammer.
If the blow be administered near the edge, instead of in the middle of the surface of the block, a somewhat similar effect will be produced, but the cone in that case will be imperfect, as a splinter of flint will be struck off, the fissure probably running along the line of least resistance; though, owing to the suddenness of the blow, the conical character of fracture is at first produced at the point of impact. This fracture will vary to some extent in accordance with the angle at which the blow is given, and the character of the hammer; but in all cases where a splinter of flint is struck off by a blow, there will be a bulb or projection, of a more or less conical form, at the end where the blow was administered, and a corresponding hollow in the block from which it was dislodged. This projection is usually known as the "bulb of percussion," a term, I believe, first applied to it by the late Dr. Hugh Falconer, F.R.S.; and on every flake, all the facets of which are purely artificial, this bulb will be found at the butt-end of the larger flat face, and the hollow depressions, or portions of depressions, on all the other facets. If on a splinter of flint such a bulb occurs, it proves that it must have resulted from a blow, in all probability, but not of necessity, given by human agency; but where the bulb is on the principal face, and analogous depressions, or portions of them, are visible on the several other faces, and at the same end of a flake, all of them presenting the same character,
- ↑ I first learnt the art of producing these cones from the late Rev. J. S. Henslow, F.R.S., and have since then instructed many others in the process, among them the late Dr. Hugh Falconer, F.R.S., whose account of the manufacture of flakes ("Palæont. Mem.," vol. ii. p. 605) is, I find, curiously like what I have written above, He insists rather more strongly on the different characteristics of "iron-struck and "stone-struck" facets than I should be inclined to do. There is, however, in all probability a difference in the fracture resulting from hammers of different degrees of hardness and elasticity The mechanics of the fracture of flint have also been studied by the late M. Jules Thore, of Dax. (Bull. de la Soc. de Borda, Dax, 1878.)
