These papers include diaries (1777-1782), expense books (1794-1804), notebooks, a commonplace book, drafts of essays, excerpts from minutes of Friends' Monthly Meeting, Tortola (1741-1761), newspaper clippings, sketches, and correspondence. Also included are letters and papers of Mrs. Thornton.

The manuscripts of father William Mitchell and daughter Maria Mitchell are not easily separated, and are treated here together. The collection includes William Mitchell's autobiography, his memoir of Judge Walter Folger of Nantucket, astronomical and meteorological observations, lectures, family and other correspondence.

After a brief career in the Congregational church, J. Peter Lesley (1819-1903) left the ministry in 1852 to work full-time as a geologist. Having gained experience in structural geology and stratigraphy with the First Pennsylvania Geological Survey (1836-1842), Lesley became an expert in the geology of coal, working for the Pennsylvania Railroad and other corporations and conducting some of the first systematic studies of the state's hydrocarbon resources. A long-time professor at the University of Pennsylvania, he was elected to the APS in 1856, serving variously as its librarian, secretary, and vice president, and he was selected as Director of the Second Pennsylvania Geological Survey (1875-1889). The Lesley Papers include letters to and from Peter Lesley and his wife, Susan on geology, coal and iron mining, abolition, educational reform, organized charity, and Unitarianism. The collection offers important insight into academic and applied geology in late 19th century Pennsylvania, the development of the coal and iron industries, as well as into the Lesleys' progressive social and intellectual milieu. It is divided into three series: Lesley's general correspondence, his correspondence with his brother Joseph, and microfilms of Lesley's research notes.

Robert Cushman Murphy (1887-1973), one of the twentieth century's great ornithologists, was curator of mammals and birds at the Brooklyn Museum (1911-1917) and was curator of birds and later chairman of birds at the American Museum of Natural History (1917-1973). The world's foremost authority on sea birds, in 1936, he penned Oceanic Birds of South America, a two volume set. An ardent traveler and explorer, he kept copious notes of his travels, which he typed up, illustrated and had bound. The APS library owns forty-two bound journals. Murphy was made an APS member in 1946. A pioneer in the study of the ecology, a louse, fish, plant, lizard, an Antarctic inlet, spider, and two mountains have acquired his name. This collection contains diaries, journals, articles, addresses and observations of Murph;, generously illustrated with newspaper clippings, photographs, letters, sketches, maps, and charts.