106 



BJ0RN HELLAND-HANSEN 



(rep. of the "MICHAEL SARS" NORTH 



The time when the maximum of current occurs may 

 be expressed in degrees (}•) reckoned from the starting 

 point of the analysis, and found by the following formula: 



tg2Y = 



2(M.N -\- P. Q) 



(M + Q)(M — Q) — (M -y P) (N — P) 



The results of the calculations of the angles may be 

 checked by means of the equation : 



tg (a - r) 



P — N 



45. Measurements from Anchored Ship. 



Observations from the "Michael Sars" at anchor were 

 made in the Straits of Gibraltar (Stat. 18) and in the sea 

 S. of the Azores (Stat. 58) as mentioned in the preceding 

 section. The reduced velocities recorded in Table V and 

 the mean directions are represented by diagrams on pp. 

 101* and 102*. The thick lines drawn in full illustrate 

 the observations at 9 metres. Observations at other depths 

 are denoted by crosses, with numbers for the depths. 

 The thin lines marked "M. S." show the true (not mag- 

 netic) heading of the ship. 



At both stations measurements were made during 

 more than 12 hours. At 9 metres so many observations 

 were made that harmonic analysis of the semi-diurnal 

 variations can be made. As previously mentioned, the 

 deviation of the compass of the current-meter has not 

 been taken into account. 



«. Observations in the Straits of Gibraltar. 



It is well known that a surface current carries At- 

 lantic water into the Mediterranean, while an under-cur- 

 rent flows in the opposite direction and brings deep water 

 from the Mediterranean into the Atlantic. The two cur- 

 rents are clearly demonstrated by the figure on p. 101*. 



The observations from the "Michael Sars" commenced 

 early in the morning of the 30th of April, when the 

 surface current was running W., with the ship pointing E. 

 The velocity of the current at 9 metres was 40 — 50 cm./ 

 sec. or nearly 1 knot (towards W.) Shortly afterwards, at 

 about 4 o'clock in the morning, the surface current turned 

 and the ship swung round. Later on until noon the di- 

 rection of the ship remained fairly constant towards W., 

 while the current at 9 metres ran eastwards with a maxi- 

 mum velocity of 115 cm./sec. (2-2 knots) at about 9 

 o'clock. In the afternoon the ship swayed somewhat in 

 accordance with the surface current, but neither of them 

 turned round. The conditions in the early morning and 

 the early afternoon showed a marked difference which 



did not correspond to the variations in wind at this place 

 (cf. the notes on the weather, p. 54*), but indicates di- 

 urnal or other variations of a longer period than 12 

 hours. 



The measurements from the rowing-boat were made 

 at 17—18 o'clock on the 29th of April not far from the 

 place where the "Michael Sars" was anchored some hours 

 afterwards. Observations were made at 5, 20 and 40 

 metres. The velocities were almost the same at these 

 three depths and greater than the maximum velocity found 

 at 9 metres in the forenoon on the following day. This, 

 again, indicates considerable diurnal variations. 



Fig. 42 shows the variations of the N.- and E.-com- 

 ponents of the current at 9 metres. By means of the 

 smoothed curves the values of the components have 

 been interpolated for every lunar hour, beginning with 

 22 L.H. The values thus found have been used for an 

 harmonic analysis which has resulted in the following 

 equations: 



V = 22-7 — 35-9 cos 30 (H f 2) + 4-6 sin 30 (H + 2) 

 u = 46-9 — 54.8 cos 30 (H + 2) — 2-5 5//; 30 (H + 2) 

 where H means the lunar hour. 



The results of this analysis are illustrated in Fig. 43. 

 The ellipse represents the variations of a semi-diurnal 

 tidal current, which turns cum sole. It has its maximum 

 (65-5 cm./sec.) towards N 57° E (« = 33°) at 4 and 16 

 L. H. and towards S 57° W at 10 and 22 L. H. The ve- 



locity of the rest current is = \ 2272 ^ 45.92 ^ 52 centi- 

 metres per second (1 knot) towards N 64° E. This 

 current is represented by the arrow pointing towards 

 the centre of the ellipse in Fig. 43. The resultant current 

 (the semi-diurnal tidal current + the rest current) at dif- 

 ferent lunar hours is found by drawing straight lines from 

 the rear point of the arrow to the different points marked 

 along the ellipse. The maximum of this resultant current 

 appears about 4 hours after the passage of the moon, 

 with at velocity of 118 cm., sec. towards N 60° E. 



These results are, however, uncertain because the 

 observations comprise 12 lunar hours only. The curves 

 in Fig. 42 show considerable variations besides the semi- 

 diurnal one. It is very probable that variations with 

 shorter periods than 12 lunar hours exist, but there are 

 evidently variations of a longer period too. When the 

 semi-diurnal variations are eliminated we obtain a resi- 

 dual variation which manifests itself by a general increase 

 of the E. -component and decrease of the N. -component 

 during the period of observation. This variation may 

 be caused by a diurnal period in the tidal currents or 

 by meteorological changes, or both. In any case it 



