W. R. Tobler is professor of geography, University of California, Santa Barbara.
This work was supported in part by NSF Grant SOC77-00191. Computer and programming assistance were provided by R. Milfiff, N. Shagour, and V. Sefcik. T. Whittaker provided the streakline program. Dr. Pignatello kindly made available data on the movement of one dollar notes. T. Everett converted several illegible drafts in typescript.