[Illustration: GENERAL ARRANGEMENT OF MARK V. TANK--FRONT VIEW] [Illustration: GENERAL ARRANGEMENT OF MARK V. TANK--SECTIONAL ELEVATION] When the flaps were closed (see diagram showing interior of a Mark V. Tank), as they had to be directly the Tank came under close fire, the crew were in almost complete darkness, and had to rely upon their periscope or, alternatively, upon minute eye-holes (about the size of the capital O’s used in this text) bored through the armour-plating. If the fire was at all heavy the periscope was usually quickly put out of action, and the officer and gunners had only the extremely limited view afforded by these holes. [Illustration: GENERAL ARRANGEMENT OF MARK V. TANK--SECTIONAL PLAN] [Illustration: DIAGRAM SHOWING HOW THE CATERPILLAR METHOD OF TRACTION CARRIED OUT THE PRINCIPLE OF THE “LARGE-WHEELED TRACTOR.” SEE NOTE AT END OF CHAPTER II.] They were thus almost entirely dependent upon their maps, the special Tank compass, and upon the information which a preliminary reconnaissance of the ground had given them. This circumstance not only profoundly modified the training of the officers and crews, but also necessitated the organisation of what was almost a new service. This service was the “Reconnaissance” branch of the Intelligence. When the Tank Corps was ordered to take part in an attack, the Reconnaissance Staff was responsible for the preliminary survey of the proposed battle site for a report as to where and how Tanks could best operate, and finally for a series of detailed maps and sketches. In these maps and sketches the route of every individual Tank was set forth from landmark to landmark, together with the assigned objectives of each machine and the obstacles which it was likely to encounter. These maps and sketches were compiled from aerial surveys, captured German maps and documents, information gained from local inhabitants, accounts given by prisoners, the original Ordnance survey, and from personal reconnaissance. By 1918 this system had been so developed that the infantry came to rely almost entirely upon their accompanying Tanks for direction. This added greatly to the importance and responsibility of the work both of Tank Reconnaissance officers and of commanders. IV Topographical information can only be adequately conveyed to a more or less trained receiver, and it was therefore found necessary to add an elementary course on Reconnaissance to the already long list of subjects in which the members of every Tank crew must train. The crew were an assemblage of experts.