88 CHALLENGER 



memory ; every day throughout the year was now a surveying day, 

 come hail, rain or snow. 



The defence of harbours by controlled minefields was now being 

 pushed forward at ports throughout the United Kingdom. 

 Roughly, these defences consist of mines laid in exact positions 

 on the sea-bed, which can be exploded under an enemy craft 

 entering the harbour by a watcher in a control station ashore. 

 He explodes the mine when he sees, by taking a bearing and 

 range of the entering vessel, that she is passing over one of the 

 mines. A number of such mines are laid across the harbour en- 

 trance and it can be seen that the position of each mine must be 

 accurately plotted if it is to be exploded according to plan. This 

 means that a triangulation network must be set up in the vicinity 

 of the harbour entrance and transit marks connected to this net- 

 work must also be set up to guide the mine-laying vessel to the 

 correct laying position of each mine. 



So here was a huge programme of harbour defence triangulation 

 to be carried out by the three surveying ships which now re- 

 mained available at Home. Many of these harbours lay in remote 

 areas where existing triangulation stations were difficult to 

 locate ; such stations may have been established by the Ordnance 

 Survey 70 or 80 years before and the marked stones lay buried 

 some two feet or so below the surface of the ground. Descriptions 

 of the locations of these survey marks, made by the surveyors at 

 the time they were laid down, were available and it was always 

 necessary to recover two such stations if the measurement of a 

 base was to be avoided and if the network of triangulation being 

 set up was to be connected to the Ordnance Survey of Great 

 Britain, which was highly desirable. The scene, of course, had 

 changed since those descriptions were made so many years ago : 

 new woods had groAvn up and others had been felled, old buildings 

 could only be located by the ridge on the ground indicating the 

 line of the ruined walls, or new buildings had sprung up to con- 

 fuse the issue. Days were spent in digging, like treasure hunters 

 on a desert island using a sketch map made by a long deceased 

 pirate, but no successful treasure hunter was happier than these 

 naval surveyors when at last the scrape of the spade denoted a 

 flat stone, and the removal of the last handfuls of earth from the 



