SURVEYING THE ROUTE. 



19 



to keep it vertical. At other times there may be currents in 

 various directions and at various depths which show no effect 

 on the line at the surface, but cause more than the true length 



■£^j^^&^ jM 



Fig. 14.— Buchanan's Water and Mercury Piezometers 



of sounding line for the depth to be paid out. The piezometers 

 enabled the indications of the sounding line to be verified and 

 corrected when necessary. 



For the most exact determinations the two instruments are 



c2 



