26 DEEP BOREHOLE SURVEYS AND PROBLEMS 



chain on to the scroll paper winding on a shaft. As 

 soon as the feed apparatus turns backward, e.g., on dropping 

 into the borehole, the paper roll is automatically cut out; 

 the pen then indicates a straight line across, as when the 

 plant is at rest. The pen works by clockwork and in one 

 hour moves over the breadth of the paper and after auto- 

 matic reversal works back in the next hour. Thus the 

 record is a continuous zigzag line. The apparatus is 

 enclosed in a glass-topped case which permits of a constant 



Fig. 5. — Lapp's stratigraph. 



observation of the progress of the borehole respecting the 

 corresponding time. It does not cut out when the plant 

 is idle as in the case of Jahr's device, and, since this latter 

 is a check on the actual working time, it can be considered 

 that Jahr's method is superior. But it can be applied 

 to percussive boring since it works off the tool feed ; however 

 this may be a source of uncertainty since the feed is here 

 hand operated. Thus the record depends on the careful 

 manipulation of the feed which if correct, i.e., if the record 

 corresponds exactly to the progress of the hole, will give 

 uniform results with Jahr's method. Both methods lack 

 in that uniform rotation of the rods is not always obtained 

 in practice. 



The Foraky Recorder. — This stratigraph is a clockwork 

 device with paper roll and recording apparatus. The 



