The morning air is cool at the Rockville Swim Center, and the blue water of the huge outdoor pool is still, for the moment. But the Early Birds got here at 6, and team practice started at 5:30, and the young people in bathing suits, chatting quietly in the shadows, have been here for hours. Someone's mother strides purposefully to the building containing the two indoor pools. The place has a sense of waiting.

8:49 a.m. -- Great Scott, this thing is 50 meters long. You have an annex at one end for divers and at the other end for tiny tots. Looks like Lake Ontario. And up on the bank you have an entire buildingful of pools, one 25 meters long, the other 25 feet, and a whirlpool big enough for five, exercise rooms, locker rooms, all kinds of rooms.

8:57 a.m -- Here comes a team swimmers class, 30 kids in goggles lined up at the shallow end. Coach says, "First heat. Go!" and wave after wave dives in, "Go! . . . Go!" They swim up the pool in rows. Amazing. Like the Big Push on the Somme. Coach wears the staff uniform, electric blue form-fitting maillot suit, high-thigh cut, black-and-white stripe.

9:01 a.m. -- Half of the pool area is still in shadow. One mom sits in her car in the parking lot reading a novel. Another in a swimsuit lounges on the coping. Low-slung plastic ribbon chairs wait all around the pool and on the grass beyond. All empty. At the tot pool, 6-and-unders are making a lot of noise. The moms stand knee-deep in water.

9:04 a.m -- So far the action is all indoors: in the Aquacise class, gray-haired women do jumping jacks as they stand up to their waists in water. Not easy. "All right ladies," their leader says, and switches to knee lifts.

9:09 a.m. -- The team swimmers are done, and two guys wearing unlaced hiking boots reel in the lane marker floats. Giant spools. Pool rules on a large board by the gate: "Always obey guards and other members of pool staff," etc. Lifeguard David Pick, 17, of Gaithersburg stands by. Never had to save anyone, he says, but he has helped pull out some waterlogged people.

9:19 a.m. -- Four intermediates venture across the pool the short way. "It's cold," says a kid at the edge. "It's okay once you're in," says his friend from mid-pool. Kid jumps in. Seven moms sit on chairs around and about. One knits. Small children circle vaguely around them like bees.

9:42 a.m. -- "We have 50 different classes, water babies to senior citizens," says Amy Jennings, 19. She lives two blocks away. Great summer job. Water babies turn out to be children as young as 6 months. Aquatic director Mark Eldridge, 26, Bethesda, who works year-round, says the busiest time is 1 to 6 p.m. Sundays. He studied computer science, economics in college, had a job with NASA but switched because he likes kids and outdoors.

9:47 a.m. -- A Korean family dressed for shopping watches two small girls having a lesson. Getting warm here.

9:54 a.m. -- "We're gonna do two lengths and then we're gonna do our turns," shouts a girl wrapped in a towel sarong and an oversized Boston College Eagles sweater to a collection of 8-year-olds. "Go, Rachel!" shouts the teacher. "Go the whole stroke! Stretch! Open your eyes, Lil! Bottom leg back! Okay, one more time across!" Everyone talks in exclamation points because of the constant splashing. "Rachel, turn on your side, your ear isn't even in the water!"

9:55 a.m. -- The Eagles sweater girl falls in. Everybody laughs.

10:25 a.m. -- Classes done, the pool is suddenly quiet. Something is brewing: A mob of yelling kids is stacked up outside the gate. Lifeguards take their places on four raised chairs, like tennis umps. Tension rises. The whole pool area is now in full sun.

My, it's a glorious day. The sky is brilliant blue with delicious little white clouds, and small children are shrieking with delight, running stubbily over the concrete, and a soft breeze flicks the dark green leaves of the oaks and elms that line the rolling lawns. Some women lie companionably in a row on their towels and talk in low tones, each keeping an eye on some child somewhere. No one moves very fast. It could all be happening a thousand miles from the city.

10:30 a.m. -- The gate opens, and at least 200 kids pour in carrying shopping bags, airline bags, shoes, thermos bottles, towels. They're from neighboring day camps. The noise level quadruples. Good lord. Make that 300. Make that quintuples. Groups settle here and there on the coping. Quick changes. They jump into the water in droves. Someone gets pushed in. Whistle blows.

10:44 a.m. -- The pool is strictly SRO. Everybody stands there bouncing up and down. Why do people always bounce up and down in the water? Not one adult in the pool. "C'mon Valerie! Get inna water!" Valerie hangs back. Gets splashed. Screams. "I never saw so many kids," says George Shackley, 11, visiting from Milford, Del., with his grandparents. He stands on the coping looking for a space big enough to jump in. The noise never quits. A solitary mom establishes herself on the grass with radio, purse, book. George jumps in, jumps out. Holds his elbows, shivers. "There's another kid here with a suit just like mine. I'm allergic to grasses, trees, weeds, you name it." Looks perfectly healthy.

11:12 a.m. -- Another batch arrives, its teen leader carrying a tennis racket, clipboard. One kid is highly visible in the pool. Tall, lean, hard muscled, nonstop grin. Troublemaker. Ducks girls. Whistle blows. Girls gang up, duck him. Splashes water on Valerie. Whistle blows. Climbs on coping, cartwheels into pool on top of crowd. Whistle blows.

11:16 a.m. -- George is in again, out again. "It's rough in there. Kids push on ya. I'm going back in." Kids gradually leave the water, collect in their groups, talk. The pool is only half full now, but there's still not enough room to swim two strokes.

11:45 a.m. -- "Free Swim Is Over!" loudspeaker announces. "Clear the Pool!"

12:01 p.m. -- Now the pool is completely empty. Noise level is near zero. Amazing. School buses maneuver in the parking lot. "We have all kinds of activities here," says Jeannie Tippett, 28, Silver Spring, the swim center director. "Synchronized swimming, scuba, aerobics at lunch hour, lots of classes. Four of our swimmers qualified for the Olympic trials." She started coaching in college where she was studying criminal justice and took this job because she'd rather work with noncriminal youths.

12:04 p.m. -- A gray man suns himself in chair. An elderly, trim couple, expertly tanned, appropriates a pair of chaise lounges, sets them up side by side. Six adults swim at a stately pace, most doing short crossways laps. Kids romp around the slide. Gradually the pool gets busier, adults drift in from the parking lot, with steady traffic to the indoor pool building. Almost all are women. Chairs are getting hard to find. Somewhere, a radio plays Springsteen.

12:45 p.m. -- "It Is Now Break Time," says the loudspeaker. "All Those Under 18 Out of the Pool." Three boys slither out, sleek as seals, gleaming, hair slicked back. Clearly, they are regulars, all day, every day, all summer. They wait by the edge, alert as cats. Adults plow like slow freighters behind kick boards. Someone opens a soft drink can: psst-click. You're not supposed to eat or drink at the pool. The snack bar is on the lower level.

12:52 p.m. -- Tank top: "On the warm side today." Bikini: "Better'n yesterday."

1:00 p.m. -- "Break's Over," says loudspeaker. Four seconds later, there are 26 kids in the pool. "You guys, break's over!" someone shouts down to the snack bar. Two women arrive: towel bag, knitting bag, novels. They scan the area for chairs, squinting, spot a couple but notice that a smoker is sitting alongside. They head for the grassy bank. Here and there sit three middle-aged men, very separate, reading. All wear Ben Franklin glasses.

We are a little community now. Over on the grass, some teen-age girls form spokes of a wheel with their heads at the center. On the lower level some very small and very large boys are playing soccer, the younger ones fierce, the older ones laughing and gentle. A group of mothers has just taken possession of the Welsh Park Beach, a sandy area with hobbyhorses and jungle gyms near the tot pool. The women slip off their wraparounds and lie back in bathing suits, sighing gratefully. Someone is reading "Dune." Someone has a Walkman on her head. Under an awning, a very old white-haired woman, dressed to the throat, sits sternly smoking a cigarette. Two shirtless boys in raggedy cutoffs stride past carrying skateboards, another wearing a snorkel mask.

1:48 p.m. -- Break time again. Plump mom calls to plump 5-year-old, "C'mon Punkin." She holds out a huge towel and envelops Punkin, whose teeth are chattering, and hugs him. All around, little kids lie on the coping, cocooned in towels. Some very small girls saunter past in high-thigh suits, adult hairdos. They look like midgets. Amazing.

2:06 p.m. -- A bunch of smart, officious 10-year-old girls is playing a complicated game in shallow water. They correct each other. All natural leaders. Close by, some 10-year-old boys clown. One plays drunk, staggers hilariously over the edge. They all laugh and want to do it too. They line up to take turns. The act becomes a ritual, the movements as stylized as a hula. No more laughs. Then the first kid invents a new gag, stands at the edge, shoves himself into the water from behind. They all laugh.

2:39 p.m. -- Divers queue up at the high board, mostly teen-agers, some adults, a couple of wise guys trying for the biggest splash. Two lone men come in wearing soft shirts, shorts, carrying beach bags.

2:44 p.m. -- Two more men infiltrate.

2:48 p.m. -- A Swedish family establishes itself under a mimosa tree. The daughters go off to swim, dad holds their barrettes. The mailman strides up to the pool house. Bermuda shorts. Mace can on his belt. Sunsuit to slacks: "I was super efficient today. I cleaned the whole house and baked George's favorite pie and got here all before 3."

3:01 p.m. -- Hey, people are leaving. The population is definitely thinning. Only six kids jump in when the break ends. Three teen-age girls, who have been tanning for hours, sashay over to the pool, enter the water elaborately.

4:09 p.m. -- Suddenly a lot of older boys are on the scene (part-time summer jobs?). They walk back and forth in front of the girl wheel. Girls apparently remain unmoved. Now nine lone men are sitting around the pool, all reading. One reads The Wall Street Journal folded commuter-wise, though no one is within 20 feet of him. Habit.

4:59 p.m. -- Okay, there he is: the first unmistakable working businessman. Blue suit, striped tie, briefcase. He gazes about, presumably sees his family, waves, heads for the locker room. The girl wheel has broken up and two girls sit with knees drawn up, talking with two boys.

5:01 p.m. -- "Break's Over," says loudspeaker. Three small boys (see above) tumble off the coping into the water. "Oh, there won't be many more after this," says the woman at the entrance booth. "You get a couple families after supper, but it's pretty quiet from now on."

The afternoon is waning. Families are beginning to gather their things and call in their tireless swimmers. Most of the younger mothers with very small children have already gone. The tot pool and Welsh Park Beach are empty. The shadows of the treetops have reached the coping now, and the water seems a deeper blue, its depths no longer glaring and opaque but inviting, translucent. Though almost as unpopulated as in the early morning, the place has a different feel, a contented feel, sated. The shouts of the children sound somehow remote, distanced, as though they were already nothing but a memory.