Fig. 15. Potential-gradient reduction-factor station on Watson's Island, Apia, Samoa, April 1929 



Fig. 16. View of short vertical collector rod on poten- Fig. 17. Calibrating the potential-gradient recording 

 tial-gradient recording apparatus substituted for apparatus, an operation performed once each week 



bent rod (fig. 11) on November 5, 1928 unless bad weather prevented 



26 



