COMPARISON OF WIRE AND ECHO SOUNDINGS 



:i3 



the table the velocity of the wind is given in knots, and it may be assumed that when 

 it falls below lo knots the ship was being allowed to drift, as the force of the wind 

 was not strong enough to enable us to adopt the usual practice of keeping the ship head- 

 to-wind. Later, when several lines of echo soundings had been run, it was found that the 

 crossing of a previous line gave a valuable check on the accuracy of the soundings, and 

 in all the cases where two oceanic lines of echo soundings cross the agreement has been 

 excellent. 



Table of comparison between Lucas and echo soundings 



From exhaustive observations recently obtained by H.M. Surveying Ships in com- 

 paring echo and wire soundings at all depths up to 5000 m. it has been ascertained that 

 the echo method is by far the most accurate. This is principally due to the fact that 

 however carefully the ship may be manoeuvred there must be a divergence from the 

 vertical which the wire takes up. This is obvious from the above table where the depths 

 by the Lucas machine are all greater. 



Up to the limit of the shallow-water echo-sounding machine soundings were usually 

 taken every i or 2 min., depending on the scale of the survey which was being carried out 

 and on the speed of the ship, but continuous soundings can be obtained if they are 

 necessary for navigational purposes. Under normal conditions the interval between 

 soundings was lengthened as the depth increased, till in oceanic waters it reached 30 

 or 60 min., depending largely on whether a stop of about 5 min. is required for the 



