He felt a sudden pressure, and then splinters and pieces of board and fragments of tile fell on him. He heard no roar. (Almost no one in Hiroshima recalls hearing any noise of the bomb…) (1.6)
Ugh, creepy. The story begins with the six subjects' memories of the explosion and their reactions immediately following it. In addition to the subjects' specific recall, Hersey draws attention to general "collective memory" of the blast in this moment.
They still wonder why they lived when so many others died. Each of them counts many small items of chance or volition—a step taken in time, a decision to go indoors, catching one streetcar instead of the next that spared him. And now each knows that in the act of survival he lived a dozen lives and saw more death than he ever thought he would see. At the time, none of them knew anything. (1.1)
When Hersey wrote this part of the book roughly a year after the bombing, it sounds like the six subjects were still pretty haunted by memories of what had happened—and thoughts/musings about how the day could have gone differently, if things had just been different.
This especially upset Mrs. Nakamura, who remembered that in a moment of confusion on the morning of the explosion she had literally sunk her entire means of livelihood, her Sankoku sewing machine, in the small cement water tank in front of what was left of her house; now no one would be able to go and fish it out. (4.9)
Here, Mrs. Nakamura had just heard a rumor that people were going to be prevented from reentering downtown Hiroshima for a long time, which made her recall a really bizarre-o post-bomb moment in which she sank her sewing machine in her house's extra water supply in an effort to protect it from the attacks. We're never quite sure what her logic was there, and her memories don't make the reasoning much clearer… probably it's a little hard to think rationally when your city has just been blown up.
They could not move a bit under such a heavy fence and then smoke entered into even a crack and choked their breath. One of the girls begun to sing Kimi ga yo, national anthem, and others followed in chorus and died. Meanwhile one of them found a crack and struggled hard to get out. When she was taken in the Red Cross Hospital she told how her friends died, tracing back in her memory to singing in chorus our national anthem. They were just thirteen years old. (4.36)
In a letter to an unnamed person offering his thoughts about the bombing a year out from it, Mr. Tanimoto included some stories that he had heard from the day of the attack that stuck out in his mind. Unlike the people who sought to move on by burying memories of that day, Mr. Tanimoto seemed to find such memories mobilizing and powerful, because they highlighted the stakes/tragedy of letting something like that happen again.
It would be impossible to say what horrors were embedded in the minds of the children who lived through the day of the bombing in Hiroshima. On the surface, their recollections, months after the disaster, were of an exhilarating adventure. (4.41)
Here, Hersey is reflecting on how the children of Hiroshima have been haunted (or maybe not haunted?) by memories of what happened a year after the blast. He then goes on to use Mrs. Nakamura's son's essay as evidence that children's memories of the event were somehow a lot more pleasant and even "exhilarating" than one might expect from a total catastrophe.
Dr. Sasaki, who had himself suffered nothing but this last, paid little or no attention to any of these revelations. He did not follow them closely in the medical journals. In his town in the hills, he treated few hibakusha. He lived enclosed in the present tense. (5.39)
When Hersey caught up with the six subjects forty years later, he mentions that Dr. Sasaki wasn't really one for getting lost in the past or wallowing in the plight of the hibakusha. In other words, he didn't really let memories get him down, by living in the "present tense."
Sister Sasaki made a speech: "I shall not dwell on the past. It is as if I had been given a spare life when I survived the A-bomb. But I prefer not to look back. I shall keep moving forward." (5.101).
Like Dr. Sasaki, Sister Sasaki ultimately decided that dwelling on/in the past was no good. She really made a 180 over the course of he story, going from feeling pretty morbid and down on her life (and understandably so) to being positive, seeking to help others, and wanting to move forward. Not dwelling too much on the past seemed to be a key part of turning her frown upside down.
As the bearers were carrying Dr. Fujii downstairs, he stirred. Swimming up toward consciousness, he apparently thought he was being rescued, somehow, after the atomic bombing. "Who are you?" he asked the bearers. "Are you soldiers?" (5.117)
Forty years after the bombing, when Dr. Fujii was ill and had to be carried out of his house, apparently some old memories of the war seeped up to the surface, and he was zoomed back to the past, thinking he was being rescued during the war. The fact that his mind went there right away speaks to the enduring power/meaning of that time for him.
On the sea voyage, an ambitious idea grew in his mind. He would spend his life working for peace. He was becoming convinced that the collective memory of the hibakusha would be a potent force for peace in the world, and that there ought to be in Hiroshima a center where the experience of the bombing could become the focus of international studies of means to assure that atomic weapons would never be used again. (5.130)
In Mr. Tanimoto's view, memory can be super powerful. Here, it appears he was hoping that "collective memory" of the atomic bombing would help mobilize peacekeeping efforts going forward. Of course, not everyone agreed with him…
He got up at six every morning and took an hour's walk with his small woolly dog, Chiko. He was slowing down a bit. His memory, like the world's, was getting spotty. (5.196)
These are the final lines of the book, and they are pretty brutal. Hersey leaves off the story with the man who (like the author himself, perhaps) saw a real purpose in returning to memories of Hiroshima, likely out of a desire to prevent history from repeating itself. However, Hersey notes that the world's memory was "getting spotty" forty years after the event, which means the book closes on a kind of melancholy note… just not quite as freaking melancholy as the note it opened on.