244 HYDROGRAPHICAL SURVEYING [chap. ix. 



Tidal The direction and rate of the tidal streams and other currents 



streams . ^ ^ ^ 



and Sur- niust be observed. 



face Cur- This is best done under ordinary circumstances from the 

 ship at anchor by means of a current log, which is simply a 



iMg.^^ very large log-ship, and is worked in the ordinary manner, 



but with a longer interval of time. The line, which is small, 



is marked at every 10 feet, and is permitted to run out for an 



even number of minutes, varying according to the velocity of 



the current. 



Then the rate per hour of the current = number of feet run 



out, divided by one hundred times the number of minutes. 



Thus, if the log-ship is permitted to run for three minutes, 



and 220 feet of line pass out, — 



220 

 Rate per hour = — =0-73 knot. 

 300 



This current log should be hove at stated times, whenever 

 the ship is on her surveying ground, and at anchor, and an 

 entry made in the current log whether there is anything re- 

 corded or not, as negative results are in some ways as valuable 

 as positive ones. Where the tidal range is great, and streams 

 change their direction, these observations will be made at 

 comparatively short intervals, in order to ascertain the move- 

 ment of the water at different time of the tide. Where 

 streams are strong, and of importance in navigation, assistants 

 will be sent to heave the current log from a boat at anchor in 

 different positions. 



The current log can be kept by quartermasters, with super- 

 vision. A watch or clock with a seconds-hand is a requisite. 

 In the Current Book will be entered the position, time, direction 

 of drift of the log-ship, number of minutes it was allowed to 

 run out, and number of feet of line run out, wind and force. 

 Blank columns for rate per hour, and time of tide, will be filled 

 up afterwards by the officer discussing the currents. 



When the tidal streams run with considerable force, the 

 most convenient method of observing them is to follow a 

 Kisbie life-buoy, having a weighted bucket slung about one 

 fathom underneath it. Keeping the boat close to the life- 

 buoy, and fixing at intervals by sextant angles and noting the 

 time of each fix, the direction and rate of the stream is deter- 

 mined on plotting the fixes. 



