In a small shed at the top of a 100-foot-tall steel tower deep in the New Mexico desert, Donald Hornig sat next to the world's first atomic bomb in the late evening of July 15, 1945, reading a book of humorous essays. A storm raged, and he shuddered at each lightning flash.
It was his second trip to the tower that day as part of the Manhattan Project, the secret American effort to build an atomic bomb. He had earlier armed the device, code-named Trinity, connecting switches he had designed to the detonators.
But J. Robert Oppenheimer, the scientific director of the project, had grown nervous about leaving the bomb alone. He told Dr. Hornig to return to the tower and baby-sit the bomb.
A little after midnight, the weather had improved, and Dr. Hornig was ordered down from the tower. He was the last man to leave and the last to see the weapon before it changed human history.
A little more than five miles away, Dr. Oppenheimer and others waited in a bunker to see if the device they called "the gadget" would actually go off. After Dr. Hornig joined them, he took his position for his next task: placing his finger on a console switch that when pressed would abort the blast, should anything appear awry. The countdown began, his finger at the ready.
The bomb was detonated at 5:29:45 a.m. on July 16 as Dr. Hornig and the others watched from the bunker. He later remembered the swirling orange fireball filling the sky as "one of the most aesthetically beautiful things I have ever seen."
Three weeks later, an atomic bomb was dropped on Hiroshima. Three days after that, another fell on Nagasaki.
It was the dawn of the nuclear age and also of a career that took Dr. Hornig to the White House as science adviser to President Lyndon B. Johnson and to academic eminence as the president of Brown University in Providence, R.I., where he died on Monday at 92, his family said.
Dr. Hornig worked under Johnson from 1964 to 1969, conferring with him on space missions and atom smashers as well as on more practical matters, like providing sufficient hospital beds for Medicare patients and desalting water for drinking.
He had actually been President John F. Kennedy's choice for science adviser. Kennedy had asked him to take the job shortly before his assassination in 1963, and Johnson followed through with the appointment.
Working for Johnson was reportedly not easy. The president was said to disdain scientists and academics after so many of them had voiced opposition to the Vietnam War, which made it difficult for his science adviser to lobby for them.
But when a power blackout hit the Northeast in 1965, the president turned to Dr. Hornig for guidance, as he did when earthquakes hit Denver. After Senator Robert F. Kennedy was assassinated in 1968, Johnson sought Dr. Hornig's advice on ways to detect concealed weapons.
Under Johnson, Dr. Hornig doubled the budget of what is now the Office of Science and Technology Policy, which he led, and pushed for federal research in housing and transportation. He also helped kill a proposal to put giant mirrors into orbit over Vietnam to spotlight the enemy at night.
As the president of Brown from 1970 to 1976, Dr. Hornig established a four-year medical school. He oversaw the merger of Pembroke College, Brown University's women's school, with Brown College, the men's undergraduate school. He faced student protests, including a 40-hour sit-in at Brown's administrative building, over cost cutting, minority admissions and other matters.
He met some student demands but later declared that the university would never again negotiate with students occupying a building. He described his presidency as "bittersweet."
Donald Frederick Hornig was born on March 17, 1920, in Milwaukee and attended Harvard, earning his undergraduate degree there in 1940 and his Ph.D. in 1943, both in chemistry. His dissertation was titled "An Investigation of the Shock Wave Produced by an Explosion," and he went to work at the Underwater Explosives Laboratory of the Woods Hole Oceanographic Institute in Massachusetts.
He joined the Manhattan Project after his boss at Woods Hole passed along a mysterious invitation asking him to take an unspecified job at an unspecified location. No explanations were offered, and Dr. Hornig declined. James B. Conant, the president of Harvard, helped persuade him to change his mind.
Dr. Honig and his new wife, the former Lilli Schwenk, bought an old Ford with frayed tires and puttered to New Mexico. His wife, who also had a Ph.D. in chemistry, worked for the project as a typist, and then as a scientist.
Dr. Hornig is survived by his wife, as well as two daughters, Joanna Hornig Fox and Ellen Hornig; a son, Christopher; a brother, Arthur; a sister, Arlene Westfahl; nine grandchildren; and 10 great-grandchildren. His daughter Leslie Elizabeth Hornig died last year.
After World War II, Dr. Hornig was a professor and a dean at Brown and then moved to Princeton as the chairman of the chemistry department. While at Princeton, he was on President Dwight D. Eisenhower's scientific advisory committee.
Dr. Hornig was briefly a vice president at the Eastman Kodak Company before accepting Brown's presidency. After leaving Brown, he taught at Harvard's public health school, retiring in 1990. He was one of the youngest scientists ever elected to the National Academy of Sciences.
This article originally appeared in The New York Times.