200 



HYDROGRAPHICAL SURVEYING [chap, viil 



Same 

 "Points" 

 to be used. 



Necessity 

 for Assist 

 ance. 



Distance 

 Between 

 Lines of 

 Sound- 

 ings. 



The same objects should be taken for the fix as long as 

 possible. It tends to check errors in reading off, as the angles 

 at each fix will bear a definite proportion to the last set. For 

 instance, if we are pulling off shore with both Mat and Pagoda 

 astern of us, the angle will be less each time, and a reading of, 

 say, 33° instead of 23° would be at once detected as erroneous, 

 before the disjointing of the line when the fix was plotted 

 showed there was " something wrong somewhere." 



The variation in the angles will also enable us to see if the 

 fix is remaining good. This plan also saves time in setting 

 the station pointer verniers. 



When assistants are not thoroughly used to the work of 

 sounding, it will be necessary to have two in each boat, to 

 ensure no mistake ; but when not only officers but men get 

 used to it, one officer will in most cases be able to carry on 

 the work by himself, with the assistance of a man to write 

 down for him. Now that seamen are all taught to write, there 

 is seldom any difficulty in finding one of the boat's crew — 

 the coxswain, if possible — to write down fairly. The same 

 man will steer generally, and so permit the officer to keep his 

 eyes for other matters. 



In deep water the boat must, of course, be stopped, and 

 the leadsman will only heave when told. The interval can be 

 timed by watch, or, in very open deep soundings, by the 

 Massey's log towing astern, fitted as described on p. 53. 



The distance between the lines of sounding will depend 

 upon the scale and the character of the survey, also upon 

 whether the place is inhabited or not, for where there are 

 natives information can be picked up as to shoals, etc., from 

 the fishermen. The value of this, however, largely depends upon 

 the intelligence of the informant, and often cannot be trusted. 



If the coast or harbour be unknown and the land of certain 

 geological formations, it takes a great deal of sounding to be 

 certain no stray rocks exist undiscovered ; and, as was pointed 

 out in our preliminary remarks, the majority of marine surveys 

 are not on a sufficient scale, nor will time at disposal allow us, 

 to sound so close as to be absolutely certain nothing is missed. 

 The surveyor must make up for this by keeping his eye ever 

 on the look-out for discoloured water, and by examining every 

 suspicious spot. 



