History of the Metropolitan Police Service been headed by a Commissioner, rather than a Chief Constable which is the highest rank in police forces outside London. The first Commissioners to hold the post were Lieutenant-Colonel Sir Charles Rowan and Sir Richard Mayne. When Sir Charles Rowan died, leaving Sir Richard Mayne as the surviving Commissioner, Captain William Hay was drafted in to jointly run the force with Mayne. However, because the two Commissioners did not agree on methods of running the force, since 1855 it was decided that only one Commissioner would run the force. Metropolitan Police patrols took to the streets on 29 September 1829,
History of the Metropolitan Police Service often called for assistance from the Metropolitan Police because of their detective experience. The last case of this was when the now defunct Buckinghamshire Constabulary called upon the MPS to help in the investigation of the Great Train Robbery. In 1931, Hugh Trenchard was appointed as Police Commissioner Trenchard served as head of the Metropolitan Police until 1935 and during his tenure he instigated several changes. These included limiting membership of the Police Federation, introducing limited terms of employment and the short-lived creation of separate career paths for the lower and higher ranks akin to the military system of officer