202 



HYDROGRAPHICAL SURVEYING [chap. viii. 



Sus- 

 picious 

 Ground. 



Close Ex- 

 amination 

 of a Shoal. 



Small 

 Buoy. 



Doubling 

 the 



Shoaler 

 Lines. 



It will depend upon the orders received from the chief of 

 the survey whether suspicious ground is searched at once, or 

 merely pointed out on return on board for further examination. 

 As a general rule, whenever the soundings, in pulling offshore, 

 say, decrease, it is suspicious, and the spot must be examined 

 "by intermediate lines, which in many cases should be at right 

 angles to those previously run, and looking out sharp with the 

 eye as well. 



In calm weather, when there is a tide, a sharp eye may 

 detect a pinnacle rock by the ripple it may form. 



It is in looking out for and utilising such small indications 

 that the genius of the true surveyor displays itself, and many 

 are the rocks that have been missed for want of such sharp 

 intelligence. 



A very distant object in transit with objects situated on the 

 shore at close and regular intervals, affords the most satisfactory 

 means of running lines of soundings across a shoal at distances 

 apart much closer than can be depicted on a chart on any 

 reasonable scale. Cross-transit lines to cut off the lines of 

 soundings at each end will keep the work within prescribed 

 limits. The ground can then be examined in the minutest 

 manner possible with great facility. Fixing is necessary only 

 when a shoal cast is obtained, when a buoy should be dropped 

 at once, and the summit of the obstruction felt for by the lead. 

 The further removed the distant object, the more nearly parallel 

 to each other become the lines of soundings, and therefore the 

 more effectively is the ground covered. The nearer the front 

 object, the more sensitive is the transit line. If cross-transits 

 are not available, it is frequently advisable to mark by buoys 

 the four corners of the area to be examined. 



The examination is more thoroughly carried out by sounding 

 on transit lines crossing the shoal in two or more directions, 

 cutting one another at broad angles. 



A small nun buoy, with light chain and a weight to anchor 

 it by, is useful in the sounding-boat to drop over on a shoal 

 spot, so as to guide a boat working round and round while 

 trying for still shoaler water. 



In many cases it is convenient to run double the number 

 of lines in shoal water — say, out to 7 fathoms — that are re- 

 quired in greater depths. In this case one set of lines will 



