Chiefly correspondence with Twining's family and friends, particularly his wife, Harriet Amelia (Kinsley) Twining, his father, Stephen Twining, and his son Kinsley Twining, relating to family and business affairs and his scientific interests; together with legal documents and correspondence relating to his inventions and educational, genealogical, religious and scientific interests; and correspondence with C. J. Slater concerning the Waverly Purchase in Illinois and Joseph Winchell and Company concerning land purchase in Missouri. Includes material relating to abolition, astronomy, occurence of meteors, use of the parallactic eyepiece, Civil War, Colonization Society, Free Constitutional Union, French-German War, Indians, land speculation, New Haven Association for Architectural and Rural Improvements, and temperance. Among the engineering projects on which Twining worked were canals in Pennsylvania and various railroads in New England, New York, the Midwest, and Pennsylvania.