Now, since it has been demonstratedc that the centrifugal force of bodies is equal to the square of their speed divided by the diameter of the circle in which they are constrained to revolve, you should not be surprised that the hardness of iron should be the effect of compression by the small vortexes of subtle matter, especially if you assume them to be very small, such that several million diameters of these vortexes, for example, would hardly equal a twelfth of an inch.