There are three sources, distinct in character, which supply materials whereby historians may be able to study the military characteristics of the period under discussion. The authentic Government records, existing either in printed form, as Palgrave's ‘Writs,’ Rymer's ‘Foedera,’ ‘Statutes of the Realm,’ and the like, or among the vast mass of original documents still lying unpublished in the Public Record Office, show the conditions of military service under the King. Numerous chronicles, as those of Geoffrey le Baker, ‘Annales Londonienses’ and others, the works of partisan clerks or monks, which form the second source, give a prejudiced and often an exaggerated account of the movements of Lancaster, Mortimer and the King, marked by an inaccuracy which was doubtless due to the writers' dependence upon second-hand information. The third source is that which has recently been made more accessible, though not to the extent which may be desired, by the publication of various local records in the Reports of the Historical Manuscripts Commissioners and of such useful works as the ‘Calendars of the City of London Letter Books’ and Miss M. Bateson's ‘Calendar of the Records of the Borough of Leicester.’ This new source of historical facts has an advantage over the other two in that it deals with events chronicled at the very time of their occurrence, and also places on record, without comment, mere statements of facts, including also the personal evidence of the chief actors in, and originators of, the incidents under consideration. The great historical value of this third source calls attention to the need for further publications of borough records such as the Memoranda Rolls of the City of London, a calendar of which Dr. Reginald Sharpe has now commenced. It is from these Memoranda Rolls, which contain letters from the King and from Henry of Lancaster, that the present paper attempts to throw new light upon the history of Mortimer's ascendency during the years 1327 to 1330.