PHYSICAL OCEANOGRAPHY 289 



half minutes it was hauled up in a wrecked condition. The wings 

 were battered and bent, and the compass was gone ; it was 

 clear that the apparatus had been bumping against the stones 

 on the bottom. The propeller had made 280 revolutions, 

 implying a velocity of 1 1 cm. per second (0.2 knot per hour), so 

 that the water had moved along the bottom at that rate at 

 least, probably faster, as the propeller must have revolved too 

 slowly after being injured. This separate measurement gives 

 the interesting result that there may be an appreciable current 

 even along the bottom. 



Now, in what relation do these currents stand to high and 

 low water ? The tide-tables show that at Cadiz and Algeciras 

 high water and low water on 30th April 19 10 occurred at the 

 followinp- hours : 



In the straits high water may with sufficient accuracy be 

 referred to about 5 a.m., low water to a little after 11, and the 

 next high water to about 5.30 p.m. It follows that the water ran 

 fastest into the Mediterranean about four hours after high 

 water, i.e. at falling tide, and that it ran fastest out from the 

 Mediterranean three or four hours after low water, that is, with 

 a rising tide. 



In Figs. 195 and 196 the current-conditions between the sur- 

 face and the bottom are shown, in the first for the 30th April at 

 9 A.M., when the current into the Mediterranean was running at 

 its maximum, and in the second the mean for the movements at 

 2 A.M. and at 3 p.m., when the current out of the Mediterranean 

 attained its greatest velocity. The velocities at the different 

 depths have been calculated with regard to the longitudinal 

 direction of the strait, the varying directions of the current 

 having been taken into account ; the actual velocities are shown 

 in Fig. 194. The two diagrams give a good picture of the 

 relation between the upper and the lower current in the middle 

 of the straits, the former about four hours after high water, the 

 latter three or four hours after low water. It is seen that the 

 boundary between the two currents lay at a depth of about 160 

 metres when the inflow into the Mediterranean was greatest, and 



u 



