From the April 13, 1903 New York Times:

“Trenton, N.J.–William Vallance, the famous lightning calculator, who could do any sum in mathematical calculation mentally and with but an instant’s hesitation, died here to-night, aged 30 years. About a week ago he was taken to the State Hospital suffering from a severe mental strain, believed to be the result of his juggling with figures.

Vallance could duplicate the feats of any of the lightning calculators, and then beat them all by stating instantly any desired date in history. His mind was a vast storehouse of historical data, and where he gathered it no one ever knew, as he was not a student. He could not tell how he knew history, but would rattle off fact after fact without ever making a mistake.

He could give instant answer to such arithmetical questions as multiply 389, 487 by 4,641. Feats in algebra were his delight. Despite his marvelous ability he would not put it to commercial use.”