The Black Gang by H.C. McNeile

Had the Italian come up five minutes sooner -- a minute even -- all would have been well. As it was, at the very moment when Drummond's crashing blow took him on the point of the jaw with mathematical precision, another mathematical law began to operate elsewhere -- the law of gravity. Something fell from a ceiling on to a table in the room below that ceiling, even as in days gone by an apple descended into the eye of the discoverer of that law.

The two men seated in the room below the ceiling in question failed to notice it at first. They were not interested in mathematics but they were interested in their conversation. One was the red-haired man of whom Phyllis had spoken; the other was a nondescript type of individual who looked like an ordinary middle-class professional man.