SURVEYING THE ROUTE. D 



water. Depth soundings indicate the position of banks which 

 are to be avoided in laying a cable, while specimens of the 

 bottom soil show when analysed whether there is any chemical 

 constituent that would act on and corrode the cable sheath. 

 The sounding tube is of gunmetal, with a valve inside held 

 down by a spring. The valve opens against the spring when 

 the pressure below exceeds that above, as happens when the 

 weight is descending and when the tube is forced into the 

 ooze or sand at the sea bottom. A section of the tube used by 

 the Silvertown Company is shown in Fig. 2. The sinker is 

 a round shot with a central hole through which the tube passes 

 freely. The shot is held in the position shown by a wire sling 

 (not shown) suspended from a hook near the top of the appa- 

 ratus marked in the illustration as the sling seat. The large 

 tube is for bringing up a sample of bottom water, and the three 

 small tubes underneath collect a sample of the sea bed. Water 

 passes freely through the tube during descent, the valve being 

 opened (as shown in the illustration) by the pressure below, 

 and the displaced water escaping by the holes at the upper 

 part of the tube. When the tube reaches bottom and comes 

 to rest the valve is closed by a spring above it (shown in 

 section in the illustration), the pressure under the valve being 

 removed. 



On starting to haul the tube to the surface the wire sling 

 supporting the weight is cut by a self-acting knife and the 

 weight released. This design has an advantage over others 

 in which the weight is released on striking bottom for the 

 reason that the weight is utilised in pressing the tubes well 

 into hard ground in the sea bed. The wad of clay or other 

 hard ground then causes the sample of mud to be retained 

 in the tube. When descending, the arm A is in the position 

 shown in Fig. 3, the strain on the wire keeping the lower end 

 of it hard against the pawl B and so preventing the knife at 

 the end of the arm cutting the sling. The latter rests in the 

 seating S immediately below the knife. On striking bottom 

 the strain goes off and the arm falls into a nearly horizontal 

 position as in Fig. 2, thus releasing the pawl, which is then 

 pulled out of the way by a spring. When the strain comes on 

 the wire again the arm A is raised and the knife cuts the 

 sling, thus releasing the weight. It is sometimes necessary to 



