TABLE VI. Spectra of phase difference fi* and phase * in cycles^ per band (band width is 0.072 cpd = 2/month). The spectra of 

 phase difference are given separately for the first and second fortnight. 



i?e-*'»=X]^ie-*"''"*' = e-''^*Z!-R.<?'''s j = V^T 



(44) 



and so the multipath phase is rotated by the single-path 

 phase shift A(p. The measured 2. 5 square cycles in 

 the semidiurnal tidal peak of multipath phase corre- 

 sponds to an amplitude of V2x 2.5 = 2. 2 cycles, in close 

 agreement with the computed 2. 3 cycles for a typical 

 tidal current. But for the Cartesian multipath, the tidal 

 energy peak is reduced by the number of paths {n = 34 

 for Bermuda) and should no longer be discernible. So 

 the hypothesis is in very satisfactory agreement with 

 observations . 



But it turns out that the MMI propagation path runs 

 through the MODE expedition area, " the only place 



where a grid of deep-sea tidal pressure measurements 

 have ever been taken (Fig. 11). Currents can be com- 

 puted from the pressure gradients. M^ tidal currents 

 have amplitude close to 1 cm/sec, but the MMI path is 

 almost at right angles to the major axis of the tidal el- 

 lipse, and the MIMI component is small and poorly de- 

 termined (Table VIII). (A computer model of tides'^ 

 has the minor axis in opposite phase.) We expect the 

 tidal phase (Greenwich epoch °G) of maximum current 

 towards Bermuda to coincide with minimum acoustic 

 phase; i.e., 180° + °G for $(0 should be about 180° to 

 agree with MODE measurements. In fact, phases vary 

 from fortnight to fortnight; there is no resemblance be- 

 tween midstation and Bermuda. 



The variability from fortnight to fortnight of a 



244 



