198 HYDROGRAPHICAL SURVEYING [chap. viii. 



depression round it, or round one half of it, especially with a 



muddy bottom. In calm weather and tideway, the ripple 



on the surface may reveal the existence of a rock a few yards 



further upstream than the nucleus of the ripple. 



starring Jn sounding off a lighthouse situated on an isolated rock or 



Light- island, the lines may be run radiating from the lighthouse, the 



^<"^8® °^ distance being fixed by its angle of elevation, and the direction 



Ship. of each line being maintained by preserving a constant anglfg 



between the lighthovise and some distant object (the more 



distant the better). The boat then travels along the arc of a 



circle which is very " sensitive," and the further the distant 



object, the more nearly will the boat's path approach to a 



straight line. The angle is altered for each line. Sounding 



round the ship at anchor may be carried out in a similar 



manner if a distant object is in sight and its bearing observed. 



The horizontal angle suffices to keep the boat on her line, and 



obviates the use of prismatic compass for the purpose. 



Depth to The depth to which the boat soundings are to be carried 



Boat's ^^'^^^ depend upon circumstances. When soundings of over 



Soundings 20 fathoms are taken from a boat it gives a great deal of 



carried. ^ labour, unless a small sounding machine is carried. , 



When the boat gets to the end of her line, and turns to pull 

 along to the end of the next one to return, soundings should 

 still be carried on, as before. 



The method of using the station pointer has been explained 



imder the head of " Station Pointer." 



Construe- It only remains to note that it must be recollected, in getting 



sT't'^n *^^^ ^•^' *^^^ *^^® right or left angle (according to whether a 



Pointer to right-handed or left-handed station pointer is in the beat) 



ber^e^™^™ must be observed of a sufficient number of degrees to be 



measured on the instrument, if possible. If this cannot be 



got, recourse must be had to tracing-paper for plotting the 



position. 



Entering The sounding-book need not be ruled. There are several 



^n B^oiT^^ ways of writing down the objects used for fixing and the 



angles between them, but the best, if space permits, as it 



does in the sounding-book supplied by the Admiralty, is to 



put them down as you look at them, the right-hand object 



to the right, the middle one in the middle of the page, and 



the left one on the left-hand side. The hour, in Roman 



