Eight years ago in Kansas City, after Ronald Reagan had lost the Republican presidential nomination to President Ford, he told the GOP National Convention a story about a time capsule.

It was the same story he was telling Sunday night, again in Kansas City, at the conclusion of the second Reagan-Mondale debate when moderator Edwin Newman interrupted to say the four-minute time limit for his closing statement had expired.

The story, then and now, was about how Reagan had been asked to write a letter that would be placed in a time capsule and opened in Los Angeles in 2076 during the nation's tricentennial.

On both occasions, Reagan told how he had driven down the Pacific coast, trying to think of what to tell a future generation, and how it had occurred to him that "they will know all about us . . . . We know nothing about them."

Both times, Reagan said he thought about the terrible destructive power of nuclear weapons and that the future generation would know whether the weapons had been used.

"And suddenly it dawned on me, those who would read this letter 100 years from now will know whether those missiles were fired," Reagan said in 1976. "They will know whether we met our challenge."

Making the same point Sunday night, Reagan asked whether someone in a future generation would say: "Thank God for those people back in the 1980s for preserving our freedom, for saving for us this blessed planet called Earth, with all its grandeur and beauty."

The story had no concluding point, and on neither occasion did Reagan say what he wrote. But in 1976 he said that, if "we fail, they probably won't get to read the letter at all because it spoke of individual freedom, and they won't be allowed to talk of that or read it."

In 1976, in the wake of his bitterly contested loss to Ford, Reagan concluded his speech by calling for GOP unity and quoting from Gen. Douglas MacArthur, who said, "There is no substitute for victory."

On Sunday, Reagan rambled from his story to praise Vice President Bush and say that they had enjoyed a "wonderful experience" in campaigning. "We have met your sons and daughters," Reagan was saying when Newman cut him off.

In Palmdale, Calif., yesterday, Reagan said he intended to finish with a line about how the current generation has responsibility to the next.