152 DEEP BOREHOLE SURVEYS AND PROBLEMS 



use it, thus giving a constant cheap control on the progress. 

 The plunger being made of a non-magnetic alloy or lead 

 eliminates any chance disturbing magnetic influences. It 

 can be made in sizes as low as 1.9 in. for running inside 4- 

 in. drill pipe. Its greatest disadvantage is that it is inter- 

 mittent in its action, having to be hauled out after each 

 record, the clock being reset and, if necessary, the paper 

 disc changed. 



