For one fleeting instant last Saturday, when his hand left the pole and his body hung tantalizingly over the crossbar, the world around Pole Vaulter John Pennel froze in hushed expectancy. In the next it exploded with ecstatic huzzahs as Pennel flipped his body safely clear of the bar and tumbled into wood shavings below, the first person ever to go over 17 feet. The actual height, 17 feet� inch, was checked and rechecked for half an hour by nervous officials at the Gold Coast AAU meet in Miami, Pennel's hometown. The long wait precluded any greater heights that day, but a confident Pennel, who six times earlier this year had broken the world record, was not worried. He had done 17 feet five days before in practice. "Once you've got it licked in your mind," he said, "the rest is easy." The rest could be another half foot or so this year, he says, and then it is anybody's guess what the ceiling might be for a quick, powerful athlete like Pennel, who has studied and ultimately mastered the complicated gymnastics of high flying on a slingshot, fiber-glass pole.