A Southern California tradition dating back nearly 40 years died quietly when the Board of Supervisors ordered the end to monthly air-raid siren tests. Supervisor Kenneth Hahn, disturbed after hearing last week's test, called for the wailing of the 400-siren system on the last Friday of every month to stop because it gives citizens "a false sense of security." Developed in the era of long-range bombers and intercontinental missiles, the sirens were originally intended to warn citizens to go to community and backyard bomb shelters. But Hahn noted, "There is no defense against nuclear missiles with warning time of as little as eight minutes from submarine-launched missiles." The board instructed its staff to determine whether the sirens could be put to a more practical use--such as alerting residents to turn on their radios in the event of regional man-made or natural disasters.