HIROSHIMA; By John Hersey. Knopf. 198 pp. $13.95. oberts

WHEN JOHN HERSEY's Hiroshima first appeared in The New 1946, its impact was instant and massive. For one thing it took up the entire magazine, for another its powerful effect came from the simplicity of the writing, a vivid, straightforward, non-polemical reportorial account of half a dozen survivors of the first atomic bomb, of what happened to them and their city that day and in the following year. As a book it sold 118,000 copies in hard covers and 3,441,000 in paperback.

Hersey, with the help of the magazine's Harold Ross and William Shawn, had produced an American classic. There were to be other compelling books about the bomb, its history, use and effect, but none has had such a lasting impact. It reminded me of another small, but, alas, now long forgotten volume about World War I, a 112-page largely photographic book called The Horror of It with searing pictures of "war's gruesome glories" not seen since Mathew Brady's photographs of Civil War slaughter.

Hersey's verbal scenes of the atomic bomb's slaughter and destruction have turned out to be, I believe, even more compelling than the photographs both of the flattened city and of the maimed and disfigured survivors. To read the book again today, with the images in one's mind of such recent catastrophes as those in Lebanon, is to bow to Hersey's superb writing. To this day nothing tells better the horror or Hiroshima:

"Others, because of pain, held their arms up as if carrying something in both hands. Some were vomiting as they walked. Many were naked or in shreds of clothing. On some undressed bodies, the burns had made patterns -- of undershirt straps and suspenders and, on the skin of some women (since white repelled the heat from the bomb and dark clothes absorbed it and conducted it to the skin), the shapes of flowers they had had on their kimonos."

"The change was too sudden, from a busy city of two hundred and forty-five thousand that morning to a mere pattern of residue in the afternoon."

"Their faces were wholly burned, their eyesockets were hollow, the fluid from their melted eyes had run down their cheeks. (They must have had their faces upturned when the bomb went off; perhaps they were anti-aircraft personnel.)"

"In a paroxysm of terrified strength, he freed himself . . ."

"To Father Kleinsorge, an Occidental, the silence in the grove by the river, where hundreds of gruesomely wounded suffered together, was one of the most dreadful and awesome phenomena of his whole experience. The hurt ones were quiet; no one wept, much less screamed in pain; no one complained; none of the many who died did so noisily; not even the children cried; very few people even spoke. And when Father Kleinsorge gave water to some whose faces had been almost blotted out by flash burns, they took their share and then raised themselves a little and bowed to him, in thanks."

This new edition of Hersey's classic contains a last chapter, almost two-thirds as long as the original book, recounting the fate of his original six survivors during the intervening 39 years. This, too, appeared first in The New Yorker. Two of the six are dead; a third became a prosperous physician, another a mother superior, still another's story explains how the hibakusha (literally, "explosion-affected persons") finally won adequate Japanese government help after years of trial and anguish. The fourth still living repeatedly toured America in search of help for his Protestant church and then for a Hiroshima memorial; it was he who had started the effort for the disfigured "Hiroshima maidens" and who found himself "hurled along on the white water of Norman Cousins' ferocious energy."

The finale of Hersey's new chapter contains his moral, subtly yet emphatically made by juxtaposition. First there are the words on the Hiroshima memorial: "Rest in peace, for the mistake shall not be repeated." Next Hersey's comment: "Tanimoto read in the papers that the United States and the Soviet Union were steadily climbing the steep steps of deterrence."

Then, here and there among the stories of these six person's fates, Hersey interposes, in italic sentences, the simply stated facts of nuclear escalation, American, Russian, British, French, Chinese and Indian.

Finally, in the last sentence of the book, Hersey's description of Tanimoto, now in his seventies, encapsulates the author's point: "His memory, like the world's, was getting spotty."

John Hersey, the prolific author of 20 books ranging from such pre-atomic wartime tales as A Bell for Adano to his current novel of China, The Call, has demonstrated in this now completed version of Hiroshima that he remains one of the most magnificent and powerful writers of modern times. This little book brings the central problem of human existence today down from the often stupifying technical and theoretical to a reality no one can miss. You must not fail to read it.