Recently Jim Propp mentioned "Infinite Powers", which is a nice easy reader with historical detail and relevance via the impact of the infinity principle on the current era. In the last chapter on "The future of Calculus", Strogatz describes the pendulum phase portrait as "Poincaré's picture". How should the reader take this assertion? I briefly checked "Mémoire sur les courbes...", but did not see anything recognizable as a pendulum phase portrait. According to D. Nolte "Tangled tale of phase space", only post Ehrenfest did the concept of "phasenraum" come to acceptance. It's well known from correspondence that Sommerfeld communicated with Ehrenfest on this topic, and /Atombau und spektrallinien/ (1921) contains a couple of early depictions of oscillator phase portraits. This is the earliest reference I've found so far. It is impossible to search even a decade of literature exhaustively. Any suggestions? Too bad there is no "Online Encyclopedia of Manifold Geometry". A nice service would enable searches such as "pendulum phase portrait" and return results including a few things about the history. I tried MPIM-Bonn's MAP: http://www.map.mpim-bonn.mpg.de/ but found nothing helpful at all. --Brad