CHAP, viii.] SEARCHING FOR VIGIAS 215 



of a reported danger. As a general rule, the shallower the 

 water, the more troublesome the task, and the greater the 

 necessity to use floating beacons to enable a close search to 

 be carried out effectively. 



The well-known instances of the Avocet Rock in the Red 

 Sea, and the Quetta Rock near Torres Strait, rising from 

 depths of 33 to 39 fathoms and 10 to 12 fathoms respectively, 

 are cases in point. 



Submarine banks rising from oceanic depths must necessarily 

 stand on bases many square miles in area. Of recent years 

 our knowledge of the angle of slope that may be expected to 

 occur at different depths has been much extended. Assuming 

 that coral reefs are built on submerged mountain-peaks, the 

 annexed sketch (Fig. 59) will show approximately the slope 



Fig- 59- 



:<Surface of Sea _^ 



-74 miles- 



usually found in great depths. It ^^ill be noted that as the 

 depth decreases the slope rapidly becomes steeper. Positive 

 soundings of considerable depth will cover a comparatively 

 large area, but soundings of 100 fathoms, no bottom, do 

 not assure us of anything to a certainty, except that the reef 

 does not exist within a few hundred yards of that cast. 



Banks can frequently be diagnosed from a sounding, though 

 deep, a little less than others around, and the only way to 

 make certain that a bank does not exist is to follow up the 

 direction of the slope with positive soundings. No bottoms 

 are of little value. 



From depths of upwards of 2,000 fathoms the slope is so 

 gradual that a bank rarely, if ever, approaches the surface 

 within 7 miles of such a sounding ; therefore anywhere within 

 a circle of 7 miles' radius, embracing an area of 150 square 

 miles round a bank rising from these depths, a sounding should 

 show some decided indication of a rise in the bottom. 



