another aspect of wallis's mathematical skills was his ability to do mental calculations. he slept badly and often did mental calculations as he lay awake in his bed. one night he calculated in his head the square root of a number with 53 digits. in the morning he dictated the 27-digit square root of the number, still entirely from memory. it was a feat that was considered remarkable, and henry oldenburg, the secretary of the royal society, sent a colleague to investigate how wallis did it. it was considered important enough to merit discussion in the "philosophical transactions" of the royal society of 1685.