---Volume 4 of this edition, published in the sesquicentennial year of Peirce's birth, covers the years of his tenure as part-time lecturer in logic at the Johns Hopkins University, his only period of regular academic employment. Consequently, many of the writings in this volume focus on logic and mathematics. At the same time Peirce continued his employment with the U. S. Coast and Geodetic Survey, made two further trips to Europe on official Survey business, and wrote or published some of his most important scientific papers. Volume 4 includes extensive selections from his classic report on gravity at initial stations, his studies on comparing a wave-length with a meter, and his quincuncial projection of the sphere. Among the significant logical and mathematical studies are "On the Algebra of Logic" (1880), "A Theory of Probable Inference" (1883), Peirce's notes and addenda to Linear Associative Algebra by his father, Benjamin Peirce, and a number of papers never before published. The penultimate essay in volume 4, the metaphysical lecture on design and chance, introduces topics that would engage Peirce for the next several years, culminating in "A Guess at the Riddle" (1887-1888) and the cosmological articles published in The Monist in the early 1890s.