412 



HYDROGRAPHTCAL SURVEYING [chap. xix. 



Method of -po ascertain the movement of the floating buoy, and there- 



fl-SCGrtcLin - 



ing Rate foro the direction in which the drag was carried by the under- 

 and Direc- (j^.j.gj^^^ a fix was taken to shore objects, and plotted on a 

 large-scale sheet of points, when the drag was let go free from 

 the boat. Subsequent fixes and times taken enabled the course 

 and distance of the buoy in the intervals to be recorded on tliis 

 sheet. 

 Surface ^ small buoy, weighted so as to float awash, put into the 



Current. J ' o ' i- 



Fig. 85. 



water at the same spot and time, and followed by another 



boat, afforded means of ascertaining the surface current. 

 Defects. This arrangement worked very satisfactorily altogether, but 



there are several defects in it. 



The depth of the submerged drag will not be the length of 



the line allowed, but some unknown quantity less, as will be 



seen by the accompanying sketch (Fig. 85.) 

 This must be estimated. 

 Rate less The f orce expended in dragging the buoy through the surface 

 than True ^yater, and overcoming the friction of the suspending line. 



