John L. Burgess, the first of three young men to go on trial for allegedly hurling rocks at traffic on the Capital Beltway, was convicted yesterday by a judge who called the attack "cowardly, dastardly and sneaky."

Nearly 30 people in 24 vehicles were injured in the pre-dawn mayhem, including Destiny Morris, 16, of Hagerstown, Md., who suffered a fractured skull. She has undergone intensive therapy since emerging from a six-week coma, but probably will never function above the mental level of a 9-year-old, according to testimony at the trial.

Burgess, 18, had admitted to police that he and two acquaintances threw dozens of 5- to 15-pound rocks at Beltway motorists "just for fun" last May 27. He was acquitted of the most serious charges he faced, eight counts of assault with intent to murder. But he was found guilty of one count of assault with intent to maim, seven counts of assault with intent to disable, and numerous counts of assault and destruction of property.

Burgess could be sentenced to as much as 415 years in prison, said prosecutor John Smathers. "I don't anticipate him getting anything like that, but the state will be recommending a lengthy period of incarceration. I'll probably recommend 50 years," he said.

Prince George's County Circuit Court Judge William Missouri, who began hearing arguments and testimony in the nonjury trial Tuesday, ordered Burgess jailed without bond until his Dec. 12 sentencing. Since his arrest, Burgess had been confined to his Forest Heights home under a pre-trial release program involving electronic monitoring.

The judge denied defense attorney James Joyner's request that his client be allowed to stay at home until his sentencing. "He's going to do some time," Missouri said. "He might as well start now."

Destiny Morris's father, Ellis Morris, applauded the verdicts.

"I saw the justice system at work, and it worked," said Morris, a self-employed painter. "It takes the fellows off the street who are doing this kind of junk, and maybe somebody else's daughter won't get hurt."

The chief investigator in the case, Maryland State Police Sgt. Vernon Herron, stood with Ellis Morris outside the Upper Marlboro courthouse after the verdicts were read.

"I don't know if I can ever be totally satisfied," Herron said. "No matter how many years this defendant spends in prison, when he gets out, Destiny Morris will still be the same."

Prosecutor Smathers, who called Burgess "just a cruel human being" in court, had sought convictions on eight counts of assault with intent to murder, each punishable by up to 30 years in prison. But Missouri found Burgess not guilty of those charges, saying Smathers had offered no convincing evidence that Burgess intended to kill any of the motorists.

"I would have liked assault with intent to murder, but I'm very satisfied with the verdicts," Smathers said.

Each of the felony charges -- seven counts of assault with intent to disable and one of assault with intent to maim -- carries a sentence of up to 10 years in prison. Burgess also was convicted of 10 counts of assault and battery, six counts of assault and 14 counts of destroying property.

Besides being acquitted of the assault with intent to murder charges, Burgess was found not guilty of nine counts of assault with intent to disable, six counts of assault and seven counts of destroying property.

In his statement to police, which was introduced as evidence at the trial, Burgess said he and two others began hurling the rocks at Beltway motorists near Indian Head Highway after seeing a movie and drinking two quarts of a cheap wine cooler to celebrate high school graduation.

They began throwing rocks from an overpass, then moved to the side of the highway, according to his statement. A pickup truck carrying Destiny Morris and two others was traveling about 65 mph when two or three rocks crashed through the windshield, one of them striking the girl's head as she slept. The rock throwing lasted about 45 minutes.

The two others arrested in the case, both 18, also gave statements to police acknowleging involvement in the attack, according to testimony at the trial.

One of them, Donnell R. Petite, 18, is scheduled for trial Dec. 10. The other, Maurice E. Ford, is to go on trial Feb. 25. Both are confined to their homes in Oxon Hill under electronic monitoring. Each faces nearly 100 charges similar to those filed against Burgess.