The park played an important role during World War I and was used for cavalry training. On 7 December 1915 English inventor Harry Grindell Matthews demonstrated, in a secret test on Pen Ponds, how selenium cells would work in a remotely controlled prototype weapon for use against German Zeppelins. Reporting on this story several years later, in April 1924, "The Daily Chronicle" reported that the test had been carried out in the presence of Arthur Balfour, Lord Fisher and a staff of experts. Its success led to Matthews receiving a payment of £25,000 from the Government the very next morning. Despite this large sum changing hands, the Admiralty never used the invention. Between 1916 and 1925 the park housed a South African military war hospital, which was built between Bishop's Pond and Conduit Wood. The hospital closed in 1921 and was demolished in 1925. Richmond Cemetery, just outside the park, contains a section of war graves commemorating 39 soldiers who died at the hospital; the section is marked by a Cross of Sacrifice and a Grade II listed cenotaph designed by Sir Edwin Lutyens.