San Francisco is blessed with several unique sound effects ideally suited to its sweeping views, tactile mists, and multicultural fragrances. Strolling over Russian Hill you can experience many of them at once: the hum and clang of the Hyde Street cable car; the flatted wail of the Chicago-bound Zephyr; the chimes and gongs of a Chinese funeral. Yet nothing evokes San Francisco's edge-of-the-continent mystique like its foghorns. Awakening in the middle of the night and hearing the distant, atonal moan, you find yourself in Dashiell Hammett's city, adrift in its "night-fog, thin, clammy, and penetrant, blurring the street." The sound conjures up images of slick alleyways murky with unseen dangers, of mysterious freighters emerging from impenetrable mists, of the cool gray city shrouded in its oceanside nimbus. In late afternoon the horns provide mellow counterpoint to the sight of a soufflélike fog bank drifting through the Golden Gate. And during the two or three times a year when the city is in the grip of a blistering 80-degree heat wave, the foghorn assumes all the heraldic clarity of an air-conditioned cavalry coming to the rescue.