measurements 

 to the south 

 of the Azores. 



272 DEPTHS OF THE OCEAN 



Current- current-metcrs we took regular observations at 10 metres, 70 in 



all, from i a.m. till 2.45 p.m. on the 12th June. Fig. 181 shows 

 the variations at this depth, which recall the current-lines on the 

 Ling Bank. The tidal current predominated, attaining a maxi- 

 mum velocity of 38 cm. per second (0.7 knot per hour) ; there 

 was also a general drift of the water towards the south-east, with 

 a mean velocity of 8-9 cm. per second (0.2 knot per hour). 

 Simultaneously another apparatus was employed to determine 

 the current at different depths down to 732 metres (400 fathoms), 

 the depth of water exceeding 900 metres. Some of the results 

 are represented in Fig. 182, which shows the current at different 

 depths: I, at 46 metres (25 fathoms); H. at 183 metres (100 

 fathoms) ; and HI. at 732 metres (400 fathoms). At all depths 

 the velocity and direction varied constantly,^ the changes in 

 direction being clockwise, and it is notable that the direction 

 shifted about 180'' in the course of half a tide-period. In this case 

 there is no doubt that tidal currents prevailed throughout the 

 whole body of water from the surface to the bottom ; they were 

 unmistakable even at 732 metres ; at this depth a velocity of 

 more than 27 cm. per second (more than ^ knot. per hour) was 

 once measured, showing that the tide can make its influence felt 

 down to considerable depths. This is particularly the case 

 where a plateau or ridge obstructs the passage of the tidal 

 wave ; in such places the current near the bottom is probably 

 increased. This would explain the remarkable fact that on 

 many submarine slopes and ridges no fine mud is deposited, 

 because the strong current sweeps the bottom clean. 



Another interesting result of these measurements is repre- 

 sented in Fig. 183, where the arrows show the currents at several 

 depths simultaneously: I. at 3.35 a.m., and II. at 7.12 a.m. on 

 the same date. We see that the currents set in different 

 directions at the different depths. In the upper layers the 

 direction shifted more and more to the right with increasing 

 depth, but from 100 fathoms (183 metres) down to the bottom 

 the direction was reversed. Thus the current at 500 metres ran 

 in the opposite direction to that of the upper layers, which again 

 approached that of the currents at the greatest depths. At a 

 certain moment the currents are, then, arranged in the fashion 

 of spiral staircases, the whole system turning in clockwise 

 direction from top to bottom. 



These observations in the Atlantic give rise to many inter- 

 esting ideas about the currents in the sea, and show that there 



