PROVIDENCE, R.I. — Donald F. Hornig, a scientist who served as a key figure on the Manhattan Project, an adviser to three U.S. presidents and president of Brown University, has died.

He was 92.

Hornig died Monday, his son, Chris Hornig, said Wednesday. He had lived in Providence with his wife for the past several years and had Alzheimer’s disease.

Horning, a Harvard-trained physical chemist, worked from 1944 to 1946 on the Manhattan Project at Los Alamos Laboratory, which developed the atomic bomb during World War II.

He was one of the youngest group leaders and designed the firing unit that triggered the implosion of the bomb’s plutonium device.

Hornig sat in a tower with the bomb the night before the first test of the weapon amid a thunder and lightning storm.

In a 1968 interview that is held at the Lyndon B. Johnson Library, he recalled the moment the bomb was detonated.

“The minute the firing needle dropped off and I knew it had detonated, I dashed out the door in time to see the fireball rising into the sky,” he said, later continuing, “I was awestruck, just literally awestruck. This thing was more fantastic than anything I had ever imagined.”

After the war, he joined Brown as a chemistry professor in 1946. He moved to Princeton in 1957.