Tracy said, in so many words, that they had not speculated about that issue, and that he tended to view it as a matter of degree rather than of kind. Small numbers of people once traveled laboriously by covered wagon; now great numbers travel effortlessly, at immense speed and often for very trivial reasons, by jet. A supersonic jet is just another point on a continuum, not nearly as far removed from the subsonic jet as the subsonic jet is from the wagon. Still, the continuum of modes of travel is not unbounded, nor is it linear. We are already close to one of its boundaries, namely, certain physical limits on the practical speed of atmospheric travel. Even if supersonic flight were no more costly than subsonic - the proposers of supersonic civil aircraft claim that it will not be - there are certain difficult obstacles, like airframe heating, which starts to be a problem around Mach 2, and the persistence of pollutants dispersed extremely high in the atmosphere, that must be weighed against the expected benefits. We are already going pretty fast, and the faster we go, the less time we save by going still faster. No matter how you value time, and no matter how much of a boost its value receives from the egos of the people who feel the strongest imperative to save it, there is certain to be a point at which the technical, environmental and economic impediments to greater speed outweigh the gain of an hour, or 30 minutes or 10 seconds.