MARKING SURFACE CURRENTS 17 



plan is to take angles between marks ashore. These 

 soundings, when they have been reduced to a uniform 

 standard, are plotted down. While all this is in pro- 

 gress, prolonged observations, which often extend over 

 a month at a time, are being made ashore of the local 

 rise and fall of tide. This is necessary, because all 

 the soundings, which obviously must be taken at all 

 states of the tide, have to be reduced to the safe 

 standard of mean low-water mark. 



Besides all this, the set and rate of the local 

 surface currents have to be determined. This is done 

 by some sort of ''current log," of which the simplest 

 form is a broad, thin plank, shaped something like 

 the cross-section of a boat, and so weighted as to 

 float perpendicularly in the water. When the ship is 

 at anchor the current log, attached to a graduated 

 rope, is allowed to drift for a given time : its direction 

 is noted, and the amount of rope paid out in the 

 given time is observed, and from these data, repeated 

 and corroborated, we get a rough-and-ready know- 

 ledge of the surface currents that is quite sufficient 

 for the practical purposes of navigation. 



When all the information collected by the topo- 

 graphers and sounding-parties and tide-watchers has 

 been united, verified, standardised and fair-copied, the 

 result is a chart ; and from all the other kinds of 

 local information that will naturally be picked up on 

 all sides by observant surveyors in the course of 



B 



