A 1 1/2-page letter from Albert Einstein to President Franklin D. Roosevelt, dated Aug. 2, 1939, and written to alert the president to the possibility "of the construction of extremely powerful bombs," was sold to publisher Malcolm Forbes today at Christie's for $ 220,000. The price was a record for a 20th-century letter.

The letter, typewritten on plain bond paper, is the shorter of two versions prepared for submission to Roosevelt, and in the end the other version was sent. That letter is part of the permanent collection in the Roosevelt Library in Hyde Park, N.Y. Along with the letter sold today was a handwritten note written in German by Einstein to e'migre' physicist Leo Szilard, who had initiated the idea for the letter, recommending the longer version. The note is heavily creased, and the rusted impression of a paper clip punctuates the upper left hand corner.

The letter set off a slow chain reaction, beginning when Roosevelt established the Briggs Committee, a forerunner of the Manhattan Project.

Forbes bought the letter for the Forbes Magazine Galleries, located at his headquarters in New York. "We were prepared to go higher," Forbes said. "It's just like a painting -- you never know if some Japanese gentleman had decided to go after it. It only takes two to make a horse race. We were glad we didn't have to go higher."

Forbes described the letter as the "embryo" that eventually led to the magazine collection's "Enola Gay log," which was kept by one of the copilots who bombed Hiroshima on Aug. 6, 1945.

As he exited the auction room, Forbes was approached by the underbidder in the sale, who asked him to donate the just-purchased letter to an institution the unidentified bidder was representing. The publisher, who was walking over to a TV interviewer, merely smiled at the extraordinary request.

The underbidder would say only that he once worked for Szilard, a key member of the Manhattan Project.

In 1939 Szilard, keenly aware of recent experiments in Berlin on nuclear fission, tracked Einstein down at his summer retreat on Long Island, where the "father of modern physics" sailed and played chamber music with his neighbors. Accompanied by his colleague Eugene Wigner, Szilard persuaded Einstein to write to Queen Elizabeth of Belgium, a friend of Einstein's, asking that her country hoard uranium stocks from mines in the Belgian Congo for use by the United States.

Einstein dictated the letter but Szilard decided it would be more effective to approach the president. A second preparatory meeting was held, at which Szilard and Einstein were joined by physicist Edward Teller. Two versions of the letter were prepared by Szilard and sent to Einstein for signature.

Szilard, after considering Charles Lindbergh as a messenger to Roosevelt, decided on Alexander Sachs, an economist and trusted presidential adviser. The letter was mailed to Sachs on Aug. 15 but it wasn't until Oct. 11 that he showed it to the president.

Einstein never participated in the Manhattan Project, and there was official action in Washington to restrict his knowledge of it since as a German national, he was considered a security risk.

Einstein's letter was considered so important that after the war he, seated with Szilard on the porch of the summer cabin, reenacted the signing for the newsreel "March of Time."

A year after the Hiroshima bombing, Einstein told a biographer, "Had I known that the Germans would not succeed in producing an atomic bomb, I would not have lifted a finger," a sentiment close to the one he expressed to Linus Pauling: "I made one great mistake in my life -- when I signed the letter to President Roosevelt recommending that atomic bombs be made."

The morning sale at Christie's totaled $ 1,126,537. The lot following the Einstein letter, a Bible translated into the Algonquian language by Indian evangelist John Eliot, also sold for $ 220,000, a world record for an American printed book. It was printed in Cambridge, Mass., in 1663.