TORRES STRAITS 149 



avoidable losses. Each time an obstruction was thus located it had 

 to be fixed and a search with t 

 find the shortest way around it. 



to be fixed and a search with the echo sounding boats made to 



Soundings on charts show the depth of water the mariner can 

 expect at Spring Low Water, being as low as the tide is likely 

 to fall. Whenever the surveyor is sounding therefore he inust have 

 a tidepole erected so that every sounding may be corrected to 

 the height of the tide above sounding level (or 'datum') before 

 it is inked in upon the sounding boards to become part of the 

 future chart. 



The height of the established level ashore above sounding datum 

 is recorded, together with a full description of the fixed mark 

 which establishes the level ; this mark is known to the surveyor 

 as a 'benchmark', formerly 'bankmark', then being a mark cut 

 in the bank of a river so that flood waters might be gauged. If 

 any future survey is undertaken in the same area at a later date 

 a tidepole can be erected, and as soon as it is levelled down from 

 the fixed mark the height of sounding datum can be established 

 on the pole without the necessity of re-observing the rise and fall 

 of the tide for the complete period of a lunar month. 



So it is that in all the larger ports of the world benchmarks 

 have been established. In many remote native villages or in shel- 

 tered coves and bays on uninhabited shores such a mark is loiown 

 by the Hydrographic Department to exist and its description and 

 height above sounding datum may be found in the records and 

 supplied to the captain of a ship sailing to survey in the area. 

 Every one of these benchmarks shows the site of a tidal camp 

 where a small party of naval ratings has lived for at least a month 

 to watch the rising and falling of the tide throughout the lunar 

 cycle. Sometimes in recent years, sometimes well over 100 years 

 ago, these simple men, by performing their tide-watching duties, 

 have established a quantity of solid data for the study of the earth's 

 tides. When recovering an old benchmark it is interesting to 

 visualise the party of men who established it ^o years before, to see 

 their camp and imagine them sitting with their clay pipes and 

 sennit hats as they converse with their native neighbours in that 

 form of pidgin English in which the sailor communicates freely 

 with those of other tongues the world over. There are many 



