54 POPULAR LECTURES AND ADDRESSES. 



tube, inside of which is placed 

 the depth-recorder represented at 

 Fig. n, for showing the depth to 

 which the sinker goes. As the 

 sinker descends the increased pres- 

 sure forces the piston D up into an 

 air-vessel, while the spiral spring 

 pulls the piston back. The amount 

 that the piston is forced up 

 against the action of the spiral 

 spring depends on the depth. The 

 marker C is used for recording 

 the depth. As the sinker goes 

 down, the 'marker is pushed along 

 the piston-rod. When the recorder 

 is brought to the surface of the 

 water, the piston comes back to 

 its original position, but the marker 

 remains at the place on the piston- 

 rod to which it was pushed. The 

 depth is read off by the position of 



FlG> ' 0613111 the cross wire f tlie mai ~ker on the 

 scale of the piston-rod.] 



