Whatever else might be said about last week's Democratic National Convention, history will have to record that, in the gathering's final hour, the Democrats utterly failed to get the balloons down or the arms up.

Convention planners from the Democratic National Committee had worked for days to set up a traditional bit of hoopla -- a multicolored downpouring of balloons during the convention's closing demonstration. And ranking Democrats from Jimmy Carter on down had worked to arrange a more important part of the pagentry -- the classic scene where the nominee and the person he defeated raise their hands aloft together in a gesture of solidarity.

But Thursday night neither event turned out as planned.

A Democratic "balloon squad" put in dozens of hours during the week blowing up several thousand balloons. Once inflated, they were stored in two soft-plastic containers, each about the size of a railroad hopper car, which were hoisted to the top of the hall before the final session began.

The balloon containers had bottoms of loose netting. According to the script, someone was supposed to yank a rope at just the right moment, pulling away the netting and releasing a grand shower of balloons.

But when the ropes were yanked Thursday night, the netting stubbornly refused to give way. Repeated yanking and tugging managed to shake down a piddling, intermittent stream, but thousands of balloons remained trapped all night.

This failure occasioned chuckles from the TV commentators and ridicule from delegates on the floor. "If they can't get the balloons out," said a woman in the New York delegation, "how are they going to free our hostages." Other delegates noted acidly that the Republicans had successfully pulled off two balloon drops at their convention in Detroit.

The national committee tactfully declined to say who or what was responsible for the nondrop, but a veteran convention organizer explained that balloon-drop engineering involves a fairly delicate balance. If the netting up there is too loose, the balloons might shake themselves out at some solemn moment of the presidential nominee's speech. If it is too tight, there's no drop.

The balloon failure deflated a demonstration that was already somewhat limp.

When President Carter finished his acceptance speech, the band played and the delegates jumped and screamed in standard fashion -- but only for a few minutes. Six minutes after the demonstration began, Walter Cronkite told his audience that "the crowd has not risen to the occasion."

This was partly because the crowd, judging from the "We Want Ted" chants that popped up now and then, was anxiously waiting Edward M. Kennedy's arrival on the podium.

Also anxious were Carter campaign officials, who had been negotiating with the Kennedy people for two weeks to make sure that Kennedy would make the traditional "unity" gestures at the convention's end.Even more anxious was David Hume Kennerly, the former White House photographer, who was assigned to shoot the arms-raised-together picture (known in the news magazine world as an "armpit shot") that was to be Time magazine's cover this week.

Kennedy's appearance on the podium came late because of a prior agreement between the two camps. Kennedy explained that the Carter staff, knowing the networks would cut to a picture of Kennedy's car as soon as he headed for the convention hall, asked him not to leave his hotel during Vice President Mondale's acceptance speech, or the Carter campaign film that followed it, or the president's address. So Kennedy did not begin the 15-minute trip until Carter fnished speaking.

When Kennedy did arrive, wearing that familiar tight-lipped smile his traveling press corps has come to call "the smirk," he strode into the crowd of Democratic officials already on the podium, gave Carter a perfunctory shake of the hand, and walked away to the side of the platform.

There followed a comical ballet in which Jimmy and Rosalynn Carter and House Speaker Thomas P. (Tip) O'Neill Jr. (Mass.) all tried futilely to lead Kennedy back to center stage for an arms-up pose with the president.

When Kennedy went to the left side of the platform to raise a fist toward his Massachusetts delegation, Carter made a beeline to join him and struck the same pose. But Kennedy's arm had come down a split-second before Carter's shot up.

On Friday, Kennedy denied that he had "snubbed" the president and said the whole mixup was due to his surprise at finding the platform jammed when he arrived.

"I couldn't believe," Kennedy said, "when we came around the corner and saw that podium . . . it was just chaos up there."

Among those on the platform was Kentucky Gov. John Y. Brown, who endorsed Carter last fall shortly after Kennedy appeared at a fund-raiser for him. "When I saw John Y. Brown," Kennedy said, "I thought, well, that's it, it's -- time's up."

Kennedy also claimed he tried to get together with Carter but "he'd be walking around up there . . . he'd be wandering around, and keep waving."

The Time photographers said they got lots of pictures of wandering and waving and the frosty-looking Carter-Kennedy handshake, but not the shot planned for the cover. "I don't think a handshake picture conveys the same message," a Time editor said.