THE BAHAMA ISLANDS 107 



likewise with A down; next reverse polarity of both needles, reversing 4 

 first; then dip observations with 4, B down, and finally 3, B down; or (b) 

 complete dip observations with needle 3, A down, and then B down; next 

 relative intensity observations prescribed in I), and finally complete observa- 

 tions with needle 4, A and B down. 



It need not, of course, be pointed out that neither the bar magnets, nor 

 any other magnets or needles, can be kept inside the tent during these obser- 

 vations nor during any of the magnetic observations. They should be kept 

 at least 15 paces away and protected from the sun or rain with the tent cover 

 and duck cloth provided. 



D. RELATIVE INTENSITY AND DIP OBSERVATIONS. 

 (All observations should be made with footscrew A towards south.) 



These are to be made with old needles 1 and 4 of 56 and the method is 

 the same as already shown and prescribed by the specimens. The needles are 

 never to have their polarities reversed. Furthermore, every care must be 

 taken so as to avoid change of the reduction constant needed to convert the 

 relative values obtained into absolute ones. This necessitates not bringing 

 the needles in the vicinity of any magnets or iron masses or exposing them 

 to sudden temperature changes, as this would alter the distribution of their 

 magnetism and thus affect the constant. They should be placed in their box 

 with opposite poles next each other and should not be inside the dip circle 

 box, if the reversal of the regular dip needles is made by mounting the revers- 

 ing block on top of this case. Above all, these needles, as well as the dip 

 needles,' must be wiped dry after each usage and be kept in a dry place, so 

 as to prevent rusting of blade and especially of pivot. 



The order will be as follows : Magnetic prime vertical determinations 

 with. No. 1; next (a) dip observations with loaded needle 4, noting time, 

 temperature and quadrants in which the two ends of the needle swing; (&) 

 No. 4, put in brass shield with face (side with A and B) of needle turned 

 towards the vertical circle and No. 1 suspended, then dip observations, again 

 noting time, temperature and quadrants, made with microscopes direct and 

 reversed for position of circle E, face of No. 1 east, and for circle -W. face W. 

 (c) Do not touch No. 4 but turn No. 1 around on the agates, so that with 

 circle W. face of No. 1 will now be east, observe with microscopes direct 

 and reversed and do the same with circle E. face W. Record time and tern- 



