Our course hitherto has been a detail of facts; now we enter upon the region of speculation. In the former Parts of this work, the history of the Roman occupation of Britain has been briefly told and an attempt made to depict the present condition of the Vallum and Wall, with their camps, castles, and outworks; now the question must be put—Is the Barrier the Work of one master-mind, or are its several parts the productions of different periods, and of different persons? Had the statements of the ancient historians upon the subject been explicit and consistent, the inquiry would involve simply an appeal to their authority; unhappily, the information which they afford is not only very meagre, but of a character so unsatisfactory, as to compel us to sift their evidence, and to compare it with the facts which we glean from an examination of the fortifications themselves. [Sidenote: AGRICOLA’S WORKS.] Agricola, we are informed by Tacitus, erected forts both on the Lower and Upper Isthmus; we are nowhere told that he drew walls, whether of earth or stone, across either of them. The northern rampart of the Vallum has by many been conceived to be the work of Agricola. In the absence of any direct historical testimony bearing upon this subject, the circumstance that the lines of the Vallum pursue a course precisely parallel to each other, must be considered as fatal to this theory. It is altogether incredible, that two engineers should at different periods construct independent works, without crossing each other’s ramparts. In Roy’s Military Antiquities, several instances are given where the trenches of one encampment cut arbitrarily those of another, the troops who last occupied the post, not seeming to pay the least attention to the works of their predecessors; the lines of the Vallum would doubtless exhibit the same appearance had they been the works of different periods. The claims of Agricola to the authorship of any part of the Vallum may therefore at once be set aside, and the inquiry be confined to the relative claims of Hadrian and Severus. [Sidenote: HADRIAN AND SEVERUS.]