Correspondence with Brian Marsden of the Yale University Observatory, and subsequently the Smithsonian Institute, regarding many topics in the field of cometary orbits, including Comets Harrington-Wirtanen, Tempel-Swift, Halley and many others; cometary calculations by the IBM-650 electronic computer, with the program according to Väisälä's method; the Newton-Raphson method for the solution of Kepler's Equation; computer calculations of nearly parabolic orbits; the designation of comets by a Roman numeral or otherwise; non-gravitational forces on comets; working with Elizabeth Roemer at Flagstaff; Dr Marsden's membership of the International Astronomical Union; Porter considering passing on the responsibility for the Royal Astronomical Society's comet reports and Dr Marsden taking this work on; matters arising from the B.A.A. Handbook; the 'Comet Catalogue' proofs in 1961; the solar eclipse of 20 July 1963; the situation in Leningrad; and Russian and Chinese astronomy in December 1967.