Philadelphia's newest mode of transportation will open for business Saturday morning, and it's unlike anything we've had before. It won't clog the streets. It picks you up at one terminal - then drops you off 15 minutes later at the same stop. It's no threat to SEPTA.

It's Zooballoon, a giant helium-filled balloon ride that will hover 400 feet over Fairmount Park four times an hour, if the wind is right.

Zooballoon is the Philadelphia Zoo's replacement attraction for the zoo monorail that delighted visitors for almost three decades. The monorail, aging and past its prime as a major draw, was dismantled four years ago.

The monorail was a serene way to take a break at the zoo by gliding above its acreage. Zooballoon - officially "Channel 6 Zooballoon," because WPVI-TV is the zoo's partner - is an exciting way to take a break at the zoo and look over the entire region. On a really clear day you can't see forever, but the balloon's operators say that you can see to the Shore, which, for many Philadelphians, is as good as forever.

Compared with almost any other Philadelphia project in recent memory, Zooballoon was built with, forgive the dangerous analogy, lightning speed. As the story goes, a brochure about balloons came across the desk of the zoo's chief operating officer, Joseph D. Moore, who liked the idea.

The zoo administration considered it last summer, and construction began in January.

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The balloon, tethered to the zoo by an underground pulley system, is a novel idea. It is only the third public balloon ride at a United States attraction; the others are at Niagara Falls and Baltimore's Inner Harbor.

Zooballoon fills a tourism void in the city, which has no real observation tower, a boon to such cities as Boston, Toronto, Seattle and Sydney, but an admittedly less attractive idea since Sept. 11. Philadelphians and tourists alike can get a spectacular view from the round walkway under the feet of William Penn atop City Hall, but the attraction is not heavily promoted. It's free, but is closed on weekends and only five people can ascend in its narrow elevator per ride.

Zooballoon can, on an optimal day - no wind - carry 30 passengers every 15 minutes. It rises not quite as high as City Hall; at 548 feet, Billy's hat is almost 150 feet higher. The balloon does go higher than the Statue of Liberty, by about 100 feet.

Its 200,000 cubic feet of helium gives it five tons of pull. "It wants to go up all the time," says Matt Monfredi, operations manager for the Great American Balloon Co., the British firm that runs the attraction.

The balloon ride isn't cheap - as a stand-alone attraction, it costs $15.95 for adults and $12.95 for children through age 11. Combo tickets with zoo admission are $22.90 for adults; zoo admission alone is $12.95. Still, "it's a very safe introduction to balloon flights, and I think it's very cost effective - a balloon flight can cost up to $200 a person," said Mike Davis, the balloon company president.

The balloon already has startled some Schuylkill Expressway drivers, who have seen the giraffe- and Channel 6 logo-bedecked globe on test flights as it glides over their cars. For zoo visitors, who'll be lining up to ride the balloon on a new plaza that replaces the monorail station, the sight of the balloon can be exciting, even scary. It all depends on your reaction to heights.

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Zooballoon's gadgetry relays wind speeds to the people who run the balloon, which is tethered by a 3/4-inch metal cable that winds on an underground winch and is held stable on the ground at 40 anchor points. Don't be surprised if the balloon is temporarily grounded; rain and snow are not problems, but shifting winds of more than 20 knots are.

Yesterday, the winds wafted at 24 knots, too bumpy for general rides but OK for experienced balloonists (unflappable), zoo workers who needed to ascend (dependable), and some reporters and photographers who asked to go (dispensable).

The zoo realizes that some people will fear the ride, and is prepared to bring it momentarily down for anyone who tries it and freaks out. In truth, the ride is gentle and the view, transfixing. Even yesterday, the ride at 400 feet was a rhythmic sway and not a scary pitch.

The skyline saluted, and the sweep of railroad tracks that shoots through the edge of Fairmount was impressive. So was the zoo property from far aloft. It's easy to see, in the open air at that height, why Fairmount Park is a city treasure, and how our rowhouses in various shades of red define us. The netting along the gondola allows riders to feel a real sense of flight, and the steel-enforced wicker outer shell gives a sense of solid support.

The ride imparts a sense of power, too, an alignment of human technical skills and nature. You know you hover over the lions and tigers below, which seem, in a strange way, like so many scaredy-cats.