This drawing has no creator identification or date, but has a note to the left of the image "The blue tint indicates the new walls. The red, the old walls, now demolished. The drab, the plan of the Dome above. The purple, that portion of the old walls retained in the new." William C. Allen (1992) The Dome of the United States Capitol: an architectural history, p. 15, 19 & 49 cites the possible significance of A. Ricard de Montferrand's design for the dome of St. Isaac's at St. Petersbourg, and his employment of iron in construction. Walter's chief draftman, A.G. Schoenborn, owned prints of the Montferrand drawings. The drawing cataloged here suggests the more information on St. Isaac was known from sources in addition to the prints, to create that color coded floor structural study. While Walter may be responsible for the study, Schoenborn & Montgomery C. Meigs, the supervising engineer, are also linked with the design & construction of the dome. Probably drawn by A.G. Schoenborn (fl. c. 1855) for Walter's studies of U.S. Capital Dome & Extensions--Laverty Forms part of: Walter, Thomas Ustick. Architectural drawings collection.