THE MUSEUM of American History has opened an eerily ordinary new exhibit on how we started hurrying and built the Bomb.

Installed in a hallway near the pendulum -- the symbolism apparently was unconscious -- the exhibit consists of a display of photographs and dreary, deadly artifacts, plus two genuine original-model atomic bombs, long since outmoded and unloaded.

Among other things, the captions tell us, we built the world's largest factory to refine and concentrate radioactive ores for the weapons; counting everyone from uranium miners to physicists, there may have been an American bomb-builder for each Japanese casualty.

Items on display from the nation's nuclear attic included a specimen of the mineral trinitite -- fused-silica slag -- more than a square mile of which was created by the Trinity bomb test at Alamagordo, New Mexico; beside it are melted roof tiles from Japan.

But perhaps the most chilling items in the exhibit are samples of the paper work that acumulated as the Manhattan Project raced to its conclusion: a carbon flimsy of thew code by which an observer radiooed baack the results at Hiroshima; and a mimeographed inventory sheet noting that 13 pounds of plutonium were ''certified to have beenexpended at Nagasaki, Japan on 9 August 1945.''