CHAP, XX.] 



ROLLER SOUNDING 



427 



the lower booms as low as possible, and the block for the depth- 

 Une as high as possible, in order that the horizontal travel of 

 the roller from the 100-foot perpendicular position may be as 

 smaU as possible. 



Fig. 91. 



A 

 .^mcnt.as '"'^el Q ( Block on sounding spar) 



fore boo m) jLen y^Z 



F ^^~^^^^C!_ Water surface _ ////| 1\\\\\\ Water surface 



n 



If required, this can. however, be allowed for in plotting when 

 the scale of the work renders it necessary. 



As fatted in the Research, when the depth is 30 feet, the roller 

 is at a horizontal distance of 29 feet abaft the perpendicular, 



