Number 141 (Story #2), August 25, 1993 by Phillip F. Schewe and Ben Stein WHAT MAKES AN HOURGLASS TICK? Scientists have discovered that the sand grains in an hourglass flow continuously only in those hourglasses with the proper ratio of neck width to grain diameter. For ratios outside the proper range, the hourglass "ticks": the grains alternate between flowing and not flowing at a constant rate. Researchers at the University of Pittsburgh (contact Xiao-lun Wu, 412-624-0873) and the University of Rennes in France have found that this "ticking" effect is caused by tiny pressure differences--typically, 1/10,000 of an atmosphere--between the upper and lower halves of the hourglass, and also by the formation of arch-like structures in the grains. Small external disturbances can alter the ticking rate--or stop the flow entirely. This knowledge can potentially be exploited to manipulate granular materials in silos and hoppers, which have geometries similar to that of an hourglass. In addition to showing the importance of air in an hourglass, this experiment demonstrates the tendency of granular materials to act as a collective substance--in this case, through the formation of arches--rather than as individual particles. This collective state of matter is neither a solid nor a liquid. For example, a normal liquid would flow more slowly through an hourglass as level decreased. (X-l. Wu et al, Phys. Rev. Lett, 30 August 1993.)