said Bush. “When one has a problem before him, say the problem of how a bridge that has not been built will sway in a gusty wind, he proceeds to make a combination of mechanical or electrical elements which will act in exactly the same manner as the bridge—that is, will obey the same differential equations.” For the physicist or engineer, two systems that obey the same equations have a kind of identity—or at least an analogy. And that, after all, is all our word analog means. A digital watch is nothing like the sun; an analog watch is the memory of a shadow’s circuit around a dial.
