Faced with his first clear sign of deadlock on the mayor's jury, U.S. District Judge Thomas Penfield Jackson took only minutes yesterday to make three crucial decisions.

Should he accept a partial verdict, if there was one? Should he give a so-called Allen charge, reminding the jurors of their duty to reach verdicts on every count if they could do so? Should he send them back to their hotel for the night, or for the weekend, then start them fresh at work another day?

The choice he made -- which surprised lawyers and some trial judges interviewed last night -- was to accept the verdict on two counts, acknowledge hopeless deadlock on the others and declare a mistrial, sending the jury home.

Experts consulted last night -- including one who watched from the courtroom -- said Jackson chose so cautiously, and went so far out of his way to avoid "coercing" verdicts, that he all but guaranteed the mistrial he had worked so hard to prevent.

Yesterday's abrupt ending after 18 months' investigation, two months' trial and eight days' deliberation, the experts agreed, came because Jackson allowed the tired jury to give up -- instead of returning to deliberate another day.

Judges normally give the Allen charge, in effect a request to try harder, and then order the jury to resume its deliberations. What Jackson did instead was to ask the jurors, after the Allen charge, whether they wished to deliberate further. Their resounding and immediate answer: No, they did not.

Charles J. Ogletree Jr., a criminal law professor at Harvard University and former public defender here, summarized what many lawyers and judges were saying last night. "It seems like an incredibly significant misstep. It's an incredible waste of resources for the city and the mayor to have the trial aborted in that fashion."

The trial's tense minutes of endgame began with a 4:45 p.m. note from the jury.

"If on any count the jury is unable to reach a verdict," the note said, "what are our instructions?"

Jackson's very first proposal, when the lawyers for both sides gathered in Courtroom 2, was to tell the jurors "that they are entitled to return a verdict to the effect that they are unable to agree."

Though it is certainly true that a jury may hang, few judges go so far as to tell them so, which was the effect of Jackson's words. That alone was highly unusual, criminal lawyers said.

In consultation with defense lawyer R. Kenneth Mundy and Assistant U.S. Attorneys Richard W. Roberts and Judith E. Retchin, Jackson decided next to ask the jurors a series of questions.

Six minutes later, he had his answers. Yes, they had unanimous agreement on some counts. Yes, they would like to return a partial verdict. No, they did not believe they could ever reach consensus on the undecided counts. And no, they did not wish to resume their work.

The judge, the experts said, had to know he was on delicate ground. It was the end of a long day and a long week, and the jurors looked exhausted. One of them, Deborah Noel, had been hospitalized for stress the night before. If Jackson pushed them too hard, he ran two risks: that whatever consensus they had reached might fall apart, and that a court of appeals might reverse him for using undue "coercion" in favor of a verdict.

Coercion, in its legal meaning, is a perceived demand by the judge for unanimity -- whether the jurors are actually unanimous or not. One judge said that sometimes judges can "read from the jury that it's just hopeless" to deliberate further. "Sometimes you can just see it in their faces," he said. In those circumstances, said defense lawyer G. Allen Dale, Jackson's priority was to lock in a partial verdict.

But lawyers and fellow judges said Jackson could have accepted the partial verdicts without ending the case. It was his next decision, they said, that brought the jury's deliberations to a close.

Over the prosecutors' objections, Jackson decided to give jurors an anti-deadlock instruction known as an Allen charge (after a turn-of-the-century Supreme Court case, Allen v. U.S.) and then ask them to tell him immediately whether they wished to continue. Given the jurors' obvious state of weary frustration, experts said, Jackson should have known what his answer would be.

"He really gave them no choice," said William W. Greenhalgh, a criminal law professor at Georgetown Law School.

What Jackson needed to do, said one fellow federal judge in an interview last night, was "to tell them in a strong voice to get back there and listen to each other." The universal opinion of judges, defense lawyers and prosecutors was that Jackson did far less.

"It is your duty as jurors," Jackson told them, reading from a standard Allen instruction, "to consult with one another and to deliberate with a view to reaching an agreement, if you can do so without violence to individual judgment . . . .

"Now with that further instruction," he said, "I would like you to go back and consider whether or not . . . further deliberations next week would enable you to arrive at a verdict."

The jurors took only six minutes to make an answer: "It is our unanimous judgment that no further deliberations will result in any verdict."

"I'm prepared to declare a mistrial," Jackson said.

"So move," Mundy replied, and the deed was done.

What struck many observers as odd about the sequence was that the Allen charge is normally regarded as a powerful inducement. Some lawyers call it a "dynamite charge" for its power to break deadlock in the jury room. When read sternly to the jurors and followed by an order to resume their work, the experts said, it frequently shakes loose a conviction or acquittal.

"The reason there has been so much litigation about it is because it proves to be pretty effective," said William D. Pease, a former prosecutor.

Some of Jackson's fellow judges said it was unfair to second-guess him in the high-pressure circumstances of the Barry trial, but everyone interviewed said a more prudent choice would have been to give the jurors the Allen charge on Monday morning.

"He gave them a way out, which completely defeated the purpose of the Allen charge," said defense lawyer Bernard Grimm.