CHAP. VIII.] SOUNDING 201 



It must always be remembered that on the ordinary scales 

 used for surveying figures may look close together, and yet 

 be in nature quite far enough apart for a rock or bank to 

 exist, without giving any indication in the lines of soundings 

 passing on either side of it. On a scale of 3 inches to the 

 mile each figure will occupy a space of 50 yards nearly. 



It is difficult to ensure the detection of all hidden danger Scale of 

 except on large-scale surveys. Five line.s to the inch is about ® 

 as close as lines of soundings can be run without overcrowding ; 

 if closer lines are required, the scale must generally be in- 

 creased. Cross-lines will to some extent compensate for 

 deficiency of scale. The decision as to the scale practically 

 governs the degree of minuteness with which the ground is 

 examined. In order to realize fully the imperfection of small- 

 scale charts, it is only necessary to enlarge to the moderate 

 scale of 6 inches a chart apparently closely sounded on a scale 

 of 1 inch to the mile. The effect is striking, and shows how 

 easily a rock may have been missed. Small-scale charts are, 

 therefore, misleading in this respect. The time occupied over 

 a survey and the consequent expense varies roughly as the 

 square of the scale. 



Though insisting upon close sounding in circumstances Unneces- 

 where it is required, it should be noted that much valuable gomid- 

 time and labour may be uselessly expended in running lines of ings. 

 soundings closely in depths from which a shoal cannot possibly 

 rise without giving some indication of its presence on one side 

 or the other of lines at a reasonable distance apart. The lines 

 and the distance apart of the soundings should be spaced 

 relatively to the depth of water in order to catch these indi- 

 cations. When found, they should be examined. These 

 remarks do not apply to depths under 20 fathoms, but in 

 greater depths in surveys on comparatively large scales it is 

 possible to sound too closely, and to do so indicates want of 

 judgment and supervision. The nature of the bottom and 

 the geological character of the country must always be taken 

 into account in spacing soundings, due consideration being 

 also paid to the angle of slope which a shoal must attain to 

 escape detection in a given depth with lines of soundings at 

 certain intervals. A diagram drawn to a true scale vertically 

 and horizontally will indicate the requirement. 



