In the downtown public library basement, in the middle of the hall connecting periodicals on microfilm to Science and Technology, there's a Halsey Taylor pedestal fountain -- the best of the best -- that'll blow your mind. Approach respectfully, press the sturdy top-corner button and, bingo, huge arch, a gusher of agua that hits the center of your tongue, pouring forth a crisp sweetness as though straight from a glacial stream. This is a perfect fountain. "It's probably because of the Bubbler," says Al Huneycutt of Halsey Taylor drinking fountains, purveyor of America's finest pedestal and wall-mount quenchers. He is referring, of course, to Halsey Taylor's patented innovation, the Double Bubbler. "It provides two streams of water that meet at the top of the arch. No other fountain maker has been able to top it," he brags. He's right. If you want the coldest Halsey in town, hit the Marshalls on South Lindbergh. It's a wall-mount, and will make your cavities vibrate. Pure heaven. But best all-around is at the downtown book bank.