For all the chaos that prevails these days in New York, there is one aspect of the city which remains the very soul of rationality, and that is its street-plan. The rectilinear network of numbered streets and avenues affords instant orientation to the visiting out-of-towner. It is not, to be sure, flawlessly Euclidean in its geometry: Fourth Street and Eleventh Street, for instance, intersect in Greenwich Village, and not at infinity as one might expect. Still, even a rube from the Midwest or Scandinavia can find his way around most of the city without too much trouble.

It is only on descending into the subway system that the native New Yorker gains a hometown advantage over the tourist. The layout of the underground train lines is a topologist’s nightmare, with routes that ought to be parallel criss-crossing as many as seven times. The scheme by which the