PHYSICAL OCEANOGRAPHY 287 



decreasing. These periodic changes, between a strong current 

 running east and a much weaker one running west, are caused 

 by the tides, which are strong enough to reverse the current. 

 The tide-period being nearly twelve and a half hours, one might 

 expect the turning of the current about 2 in the afternoon ; at this 

 time it was, however, still setting east, though with comparatively 

 small velocity. It was thus only once in the day that the 

 current at 10 metres ran out of the Mediterranean; in other 

 words, there was a difference between the two tide-periods in the 

 same day. It is probably connected with the so-called "daily 

 difference " of the tide, well known in many places, which 

 manifests itself by each alternate high-water being conspicuously 

 greater than the intervening one. We must, however, bear in 

 mind that these results, of course, only apply to the particular day 

 on which the observations were made, and we must therefore 

 beware of drawing general conclusions until observations during 

 a longer period and at different times of the year are available. 



On the preceding afternoon (29th April) we obtained from 

 the life-boat some measurements of the velocity of the current 

 at a depth of 5 metres. At 5,15 p.m. the velocity was 113 cm. 

 per second (2,2 knots per hour), and was then on the increase, 

 being more than 150 cm, per second (nearly 3 knots per hour) 

 at 6 P.M., and the current then set eastwards. This corresponds 

 to the increasing velocity eastwards at a depth of 10 metres half 

 a day and a whole day afterwards. Some observations in the 

 deeper strata were also made from the life-boat about 6 p.m. on 

 the 29th April, the velocity at 25 metres being 124 cm, per 

 second (2,4 knots per hour), and at 50 metres 138 cm. per 

 second {2.7 knots per hour) ; at both depths the current set in 

 a north-north-easterly direction. Unluckily the observations were 

 then interrupted for many hours by the breaking of the anchor- 

 cables, otherwise we should have had continuous observations 

 during two whole tide-periods. 



On the 30th April we obtained some series of measurements 

 from the steamer down to the bottom in about 200 fathoms of 

 water. The current often ran so fast that the wire with the 

 apparatus was brought into a slanting position, and the first 

 messenger was not sent down for some minutes to allow time 

 for adjustment. This rendered the determination of depth 

 somewhat uncertain ; the depths quoted refer to the length of 

 wire out, and may sometimes exceed the actual depth, but it 

 was useless to apply corrections, as we did not know the lie of 

 the line in the water. Fig, 194, 2, shows the current at 46 



