ABSTRACT
If you had been casually driving through an American urban residential neighborhood of row houses one hot summer afternoon during the early 1950s with your windows down and car radio off, you might have been struck by a strange phenomenon: a noticeable humming sound as you passed by four or five houses, then silence until you reached another block of homes. After you passed three or four houses on that block, the humming sound may have returned, followed by a brief respite, and then a similar noise as you reached the last set of houses in the block. And so on as you continued through various neighborhoods. At one point, you might have driven past a block of houses and heard no humming sound at all, whereas for other blocks, the sound was virtually non-stop. If you had taken the time to investigate the source of the strange sound, the answer would have become quickly apparent-the noise of overworked air conditioning units mounted in the front window of some of the homes.
