With the renewed interest in nuclear weapons I have been struck by how few people there still are who have seen one explode. There are a few survivors of Hiroshima and Nagasaki and there are a small number who witnessed some of the above ground test explosions. But the last American above-ground test was in 1962 and the last above-ground test by any country was conducted by the Chinese in 1980. This means that the Indians, Pakistanis, Israelis—to say nothing of the Iranians and North Koreans—have never seen a nuclear explosion. In the main, this is a very good thing: the fallout from such a test is a real health hazard. But there is a downside. We have lost the experience of watching a nuclear explosion—perhaps the most powerful lesson about nuclear bombs there is...

Over the years Francis, who died in 2007, said very little about our experiences watching these two explosions at Mercury, so I do not know how they affected him. I know how it affected me. I was never quite the same. I cannot think of nuclear weapons as an abstraction. I listen to debates on nuclear proliferation and wonder if these people really understand what they are debating.