THERE ARE a few rules that must be followed when you take a ride in the new Philadelphia Zooballoon, set to open to the public this Saturday:

There is no smoking, eating or drinking.

There is no spitting or tossing items from the gondola.

You must also hold the hand rails during takeoff and landing.

Permit me to add one more:

Keep your eyes open.

If you keep them shut, you'll be missing a unique, albeit pricey, opportunity to see our city the way few have before - 400 feet up, 360 degrees around.

As a recent test ride demonstrated, the Zooballoon offers the thrill of an unobstructed, bird's-eye view of the city from a stable and comfortable perch, without inducing the heart-stopping chills of a skydive or a Six Flags Great Adventure roller coaster.

There is a gentle tug on takeoff, as the brakes on the computer-monitored ground control system release, allowing the 115-foot-high helium-filled flying machine to climb into the sky tethered to an unfurling, 98,000 pound steel cable.

As the balloon climbs, first slowly, then at a comfortable 100 feet per minute into the cooler upper atmosphere, there is a pleasant sensation of floating.

Depending on the breeze (the ballon does not fly when winds exceed 25 knots), there can also be a gentle sway to the circular, steel-panelled and nylon mesh gondola.

The "basket" is capable of holding 30 passengers, who are welcome to walk around the perimeter for different views and snap pictures during the 15-minute ride.

The balloon does not fly, per se - it is attached. Ten redundant computer and manually operated safety features are in place to make sure that what goes up, also comes down as gracefully as it ascended, right on target, said Matt Monfredi of the Great American Balloon Co., which has installed similar rides all over the world.

A trained zoo employee accompanies each ascending group, offering information about the balloon, the zoo and the sights below.

This being Philadelphia, zoo officials also took into account urban safety issues surrounding the possibility of the balloon being struck by gunfire.

"It is filled with 200,000 cubic feet of helium - it's not the equivalent of popping a balloon at a birthday party," explained zoo Chief Operating Officer Joe Moore. He said the zoo took out $12 million in liability insurance to cover the $2.5 million ride.

"We can very safely bring passengers down to the ground. It would take many hours with a couple of punctures to have a significant impact on the lift of the balloon."

Now some people don't like heights, and they should probably not ride the Zooballoon, unless they want to judge the quality of the air-sickness bags that are kept on board.

Otherwise, to cower in the gondola would be a huge waste of $15.95 ($12.95 for kids).

That's the premium price for a 15-minute ride in the balloon for a non-zoo member who does not purchase a zoo ticket.

Zoo members will pay only $9 for the balloon ride, and non-member adults and children who purchase a zoo ticket will pay $22.90 and $19.90 respectively.

Two other similar rides operate in the country - in Baltimore at the Children's Museum and at Niagara Falls in upstate New York.

The Philadelphia experience is billed as a "Soaring Safari." But due to the abundance of tree cover, only limited views of animals on the 42-acre zoo grounds are possible. The easiest to see are in the African Plains exhibit, Bird Lake and Bear Country.

In fact, the real "safari" appeal of the Zooballoon comes from viewing the modern world beyond the D-shaped zoo's walls.

In one direction is the Philadelphia skyline, which has grown up since the late 1980s, when Liberty Place became the first skyscraper permitted to surpass the William Penn statue on City Hall, 548 feet above Broad Street.

Less appealing is the tangled network of trains and tracks serving Amtrak's 30th Street Station, and the perpetually traffic-clogged Schuylkill Expressway, where there have been reports of rubber-necking during the trial runs of the balloon.

Still, the dominant impression left by a flight on the Zooballoon is how green our home really is when you're above street level.

You can see the verdant, leafy sprawl of Fairmount Park's 4,180 acres extending to the horizon on the north and west and blanketing city neighborhoods for miles.

The last zoo "amusement" - the monorail - was nearly 30 years old and in serious decline when it was discontinued in 1998.

Officials hope the Zooballoon will add to the bottom line by attracting 75,000 riders a year, although the steep a la carte cost of the ride makes it hard to imagine a good deal of repeat business.

But for a city that has always been experienced from the ground up - a city without a Space Needle, Empire State Building or Sears Tower - the view from above is great.

Just remember to keep your eyes open.