William S. Ferguson, McLean Professor of Ancient and Modern History, Emeritus, and former Dean of the Faculty of Arts and Sciences, died yesterday at the age of 78.

A member of the Classics and History department of the University from 1908 until 1945, he was the first professor of Ancient History in the United States.

During his term as dean from 1939 to 1942, he helped to reorganize the system of faculty competition, promotion, and tenure. Previously, President Conant had appointed him to a Committee of Eight which studied faculty personnel problems.

Ferguson was will known for a theory of Greek history called "Ferguson's Law," originally presented in his graduate thesis at Cornell University in 1898. This theory, proved correct by later discoveries, provided a revised chronology for some 300 years of Greek history.

His other books dealt chiefly with ancient Athens. "Hellenistic Athens" (1911), "Greek Imperialism" (1912), several chapters of the Cambridge Ancient History, "The Treasures of Athens" (1931), and "Athenian Tribal Cycles (1932) are among his most significant works.

One of his better known doctoral students was President Pusey, who received his Ph.D. in 1937 for a study of Athenian law.