Mild-mannered Thomas A. McMahon spends most of his time as McKay Professor of Applied Mechanics. But once every ten years or so, he ducks into a Pierce Hall phone booth, and emerges as Thomas McMahon, author.

He is somewhat self-conscious being interviewed as a writer. "I only have two professions for two weeks in an interval of ten years," he claims. "In between everyone forgets about me." His new novel McKay's Bees is "a pretty well kept secret," he says.

In the interim--his last novel, Principles of American Nuclear Chemistry, was published almost ten years ago--McMahon keeps up appearances as a scientist and teacher. His field is biomechanics--how people work as machines, why animals grow to the sizes and shapes they do, why trees assume graceful shapes. McMahon designed Harvard's indoor track, which features a specially cushioned surface that improves a good runner's mile by as much as several seconds.

How does fiction fit into all this? Writing fiction, McMahon claims, is a lot like doing science.

"They support each other in a strange sort of way: One goes through the same stages--you get an idea, and you use your education or your experience to test it," he notes. Both good science and good writing demand imagination, McMahon says, but "the ideas come for nothing, or as a gift." The work comes in testing the ideas: "The ideas themselves aren't worth anything, until they are proved true or false. Just as experiments or theories test scientific ideas, a fictional idea can be proved workable or useless by trying it out."

Methods aside, McMahon draws on a more direct connection between his work as a professor and his writing. Gordon McKay, protagonist of McKay's Bees, is a familiar name in Harvard science departments. About 50 scientists, including almost the whole faculty of Engineering and Applied Sciences Department, owe their livelihood to his very large endowment. McMahon, one of the flock, pays tribute with his novel--"90 per cent of the book is lies about Gordon McKay," he says, though the last chapter, in which McKay returns to Cambridge, makes a fortune in shoe manufacturing, and befriends several Harvard faculty members, is "mostly true."

McKay is not the only half-truth in the novel; Louis Agassiz, a Harvard professor in the 1850s and the national biologist in a golden age of zoology, plays a small but acidulous part in the book. "I have been accused of character assassination," McMahon says, "but in fact his character is a lot worse than I said. He was famous for exploitation of the young people in the museum, for signing his name to their work. The accusations came so credibly and so often, that even his biographer concluded there is a lot of truth to them."

If McMahon includes a fair amount of genuine historical detail and character, he traces the fictional ideas to his own experience: "Fiction is only half fictional all the time."

"Its material is every time and place. A writer imposes his own thoughts about friends and family and his understanding of how the world works."

Although McMahon's novels have both been well-received and a third is on the way, he insists that his true profession remains science. "One can only really have one profession, one thing that one is known for. But that doesn't prevent one from having a powerful distraction," he says.

"I've been writing fiction since I was very young, but I'm not sure there's such a profession as writing fiction. There is a very precarious niche available to five or seven people in the world at any one moment. But there isn't any secure living in it.

"You usually do something that people perceive they really need. Most people think they have enough books. Some people think they have enough science, too. But I am an applied scientist, making some sort to use out of fundamental ideas in science."