A human cannonball, or human rocket, as it is known in the space age, is usually seen moving at a speed of 75 mph through the air. It appears blurry to the human eye and does not lend itself to leisurely study. Five, four, three, two, one, BOOM!, gasp, applause and a theatrical bow is what the public generally has to settle for.

Of course, a human rocket cannot stop to describe what it is doing while it is doing it, like Julia Child explaining a porridge. Indeed not: Upon being fired, it briefly experiences a force 16 times that of gravity, which tends to clamp the jaw shut. Toward the end of the flight the G forces are reduced, but then wind noise makes even simple messages ("Feels great, folks!") impractical.

A human rocket cannot even wave as it goes by, because any change in body position would cause the projectile to alter its trajectory and possibly miss the net. Since a human rocket is designed to be used over and over again, just like the space shuttle, it must always land in the net.

But enough of dry theory: Here is Elvin Bale, 35, The World's Greatest Daredevil, who developed the Human Rocket in 1978 for the Blue Unit of Ringling Bros. and Barnum & Bailey Circus.

"We built it on a 1964 school bus frame, and it cost $40,000," said Bale, who is not at all blurred when standing still. "We shot a dummy out of it for a couple of days and then I got in. But I only went about 15 feet the first time."

"Hi, boss," said Christopher Adams, joining him in the wings of the Charlotte, N.C., Coliseum, where the circus was playing two weeks ago. Elvin Bale does not do the Human Rocket act anymore; he is too busy with his Wheel of Death, his Mechanical Monster and his high-wire motorcycle acts, so he taught it to Adams. Adams is a 27-year-old Polish teeterboard acrobat who also doubles as Bale's assistant on the Wheel of Death.

"One of the reasons I picked him was that we sort of look alike," said Bale, bowing to the dashing Pole.

Adams put his arm around Bale in a satirical manner. Bale ducked. There is no sense throwing even a friendly punch at Adams. As one of the most talented acrobats in the circus, (he can do a one-armed pushup on the head of a man standing on the shoulders of a man standing on another man's shoulders) he is very hard to hit.

"When I first started doing the Human Rocket," Bale continued, "I only went 60 feet, but the force was 16 G's. In case anybody wants to argue, Al Worden, who went to the moon with Apollo 15, figured out the G forces for me. Hotshot pilots would say, 'Bull----!, I go into a power dive and I'm pulling 8 G's and I can't stand it.' But that's for 15 or 20 seconds. This is for less than a second, and even so it compresses your vertebrae and sometimes knocks your wind out."

"I used to push the button for him," Adams said brightly.

The two cannonballs sure do make it seem like fun being cannonballs.

Adams is obviously proud of the job. "I'm his assistant," he explained. "I learn from A to Z. I saw him fly every day, and I thought it would be easy. But my first time he lets me, I flew only 15 feet, and when I land in the net I can't even talk. The whole world was spinning still."

"You have to do a somersault to land on your back," Bale explained. "But it's hard to control."

"I thought it's to be like diving in swimming pool," said Adams. "There you have control from beginning. But this, at beginning, you have no control. Only toward end of flight do you have a little control."

It is a little like ski jumping, they said. But not much. Both have done some ski jumping. They have done all of those things that come naturally to daredevils and acrobats.

"In my country I join acrobatic club," said Adams. "Make pyramids, tumbling, ski trips, all like that."

"Things like that," Bale says, "he and I can learn to do in a few hours. I'm not saying we can be champions, but we can learn to do it."

"When I first started the rocket, my back was wiped clean," said Adams. "This is because the net is so tight."

"He was lowering his head," Bale said. "If you lower your head, you don't go high enough. If you don't go high enough, you skid along the net and it takes the skin off your back."

"Inside rocket, you lie down, put your feet against wall," Adams explained. "A thing like a saddle pushes you up, woom!"

"It's a very primitive saddle, though," Bale said. "It has to be, so that your feet don't catch on it. The rocket must be powdered on the inside, there can't be any snags on your costume, and you hope it runs completely clear. If it doesn't, you only go halfway to the net . . ."

"In practice, I almost went right over," said Adams enthusiastically.

When he speaks, he tends to look at Bale, who grins at him. There is a bond between them that goes beyond boss and assistant: Perhaps it is human rocketry, or maybe just that they are both rather good looking and often followed by girls with autograph books.

"Elvin is heavier than I am, and he uses 1,600 psi pounds per square inch of hydraulic pressure ," said Adams. "So I used 1,500 psi. I was almost overshooting! The correct for me, now we know, is 1,400 psi."

The Human Rocket, which is rolled out every performance, elevated, loaded with Christopher Adams and fired, is powered by a dozen or so thick lengths of elastic rope known as shock cord, drawn and held by hydraulic machinery. When the countdown is completed, the elastic is released and the human rocket snapped like a pebble in a slingshot over all three rings of the circus. It is something to behold.

"The biggest fear," Bale explained, "is that the thing doesn't go off. Because then he has to climb out."

"Oh, yeah," said Adams. "The problem is if it goes off while you're climbing out."

"When you're in there, you're all tensed up, feet planted, waiting for the shock," Bale contributed. "But if you're just six inches from the bottom, that's enough to break your back. And if you were halfway out I'm sure it would cut you in half. It's a very powerful cannon. Put a sandbag in there and you could knock down a wall."

The former best known human cannonball is Hugo Zacchini, who was with The Greatest Show on Earth for many years. Hugo Zacchini gave the impression that he had invented the act.

"But really, it's very old," Bale said. "It was developed in England, in about 1880. They called it a 'projection unit' then, and it was done on a stage. Same type of mechanism we have today. They fired a girl up about 25 feet, and she landed on a trapeze. There were two men under her to catch her if she missed. Then they made it look like a cannon. In the early 1900s there was a woman named Zazel who was shot into a trapeze. After that, lots of people did it. What made Hugo Zacchini famous, I think, was TV.

Bale freely admits that the 175 feet Adams flies is not a record. The record, he says, is 250 feet.

"But there's a lot of respect in the circus for the act," he said. "See, when you get Evel Knievel or the stunt men in L.A., they calculate their chances and then they do the stunt once. But when you do it day by day, the odds decrease on you. In two years, we do a thousand shows. Something can always go wrong. Your mind gets tired, especially when you have a six-pack circus parlance for six shows in two days . But you still do it. I'm 35, and I feel it more and more."

In his Wheel of Death act, Bale runs precariously on the top of a sort of hamster cage, 50 feet above the tanbark, without a net. He has been doing it for years, and it is still a heart-stopping routine that is obviously not fake and that will be popular as long as he wants to keep doing it. However, because the circus is always looking for something new, he tried carrying Adams on his shoulders on the Wheel of Death last year. That lasted only a few performances.

"It was just suicide, I decided," Bale said. "With Chris up on my shoulders, there was nothing he could do if something went wrong. If I ever lost him, well, he'd have no chance. We did it in a few of the large cities--New York . . . But ah, it's just too dangerous for Pikus."

Pikus is Adams.

"It's Polish for little," Adams pipes up. "Comes from Italian, piccolo."

Elvin Bale is asked how much life insurance he carries. He says he used to have a policy for $1 million. "But then when I got divorced, I canceled it. Now I have $200,000."

As for Adams, he isn't sure he has any life insurance, but promises to look into it--right after the show.

Tomorrow: The Smallest Man in the World.