114 CHALLENGER 



ments, one in each direction using steel tapes, differed by only 

 1 1 inches, which even Commander Jenks considered good 

 going, for it was the longest base measurement in hydrographic 

 history. 



The seaward approach to the river was the first area to be 

 surveyed so that the light floats and navigational buoys could be 

 moved to mark the route that carried the deepest water across 

 the sandbanks of the estuary, where little more than 26 feet of 

 water at low tide was found. Some of the surveying marks erected 

 along the shore-line were made into permanent navigational 

 beacons so that a vessel could more easily fix herself when closing 

 this featureless coast, and so that the light floats and buoys could 

 be re-laid with ease and certainty at any time. The proximity of 

 the Vichy French was brought home when one of these shore 

 beacons had to be erected right on the border between Gambia 

 and Senegal, only five miles northwards along the coast from 

 Bathurst. On two or three occasions Vichy planes came in low 

 over the surveying boats, or circled the ship outside gun range, 

 apparently looking to see what was going on. Sometimes an 

 enemy plane would fly high over the anchorage off the town and 

 every ship would bang away with her guns until the pilot decided 

 to return northwards. 



On completion of the work outside the river entrance the 

 survey moved into the river itself, where every square yard of 

 deep water was required to be known so that anchor berths 

 could be planned and mooring buoys laid with the maximum 

 economy of space. Here the real difficulties began. Up till now 

 the surveyors had been concerned with the estuary, the shores of 

 which were solid earth, where marks could be erected and angles 

 observed with ease. The river, however, which was several miles 

 wide, had no clearly defined banks and on either hand flowed 

 through vast areas of unexplored mangrove swamp. Along the 

 fringe of the mangroves there might be from two to eight feet 

 of water over the soft muddy river-bed, depending on the state 

 of the tide. 



It was at points along the fringes of the mangroves that triangu- 

 lation marks had to be erected; they had to be over 60 feet in 

 height so that they would be visible up to 10 miles away, and at 

 the same time a firm wooden observing platform had to be sunk 



i 



