MOTION 319 



southernmost pier of the bridge in line with the pylon of the old drawbridge to the eastward of it. 

 This conveniently gave the wind direction as well. Times of arrival of specimens opposite the Parador 

 forecourt wall A were taken. Near here they were generally driven ashore. A spherical glass-float, 

 which had been put over the side at C with the specimens, then had to be sighted, chased and its 

 position approximately fixed by cross-bearings on shore marks. It consistently travelled down-wind. 

 Five runs were made on 15 April towards the end of flood-tide between 11. 1 1 and 12.35 nr - The times 

 taken for Physalia to be driven from the starting-point C to the end of wall A were as shown in Table 7. 



On this day, 15 April, the wind-velocity (? at Airport) was given by the kindness of the Meteoro- 

 logical Office, London, as 19 knots at 12.00 hr., direction 070 . 



A second series of experiments was carried out on 16 April, at slack-water (low tide), when the 



velocity of the wind was less than on 15 April, perhaps 8 knots. No data could be given by the 



Meteorological Office, except that at 09.00 hr. the wind velocity was 8 knots, direction 120 . The 



results were as shown in Table 8 : 



Table 8 



Notes on 16 April 

 A 'roll' at the start 

 Arrived 20 m. out from wall A 



Arrived close to wall A. 



Picked up again (between extremes) 



Arrived 50 m. to east of wall A. Time lost through 

 bad discipline of crew (2 boys) 



No further opportunities for similar experiments occurred before I left Arrecife on 24 May. 



The direction taken by the long axis of Physalia relative to the wind is about 40 , estimated by 

 holding close above the specimens a long pennant attached to the end of a bamboo.* It was not easy 

 to have the boat manoeuvred into the right position for close observation without blanketing the 

 specimen or impeding its progress, and my notes were more than once blown into the water. 



In order to get some idea of the angle of drift from down-wind direction at slack-water in a light 

 breeze, I select the experiments on specimens 7-10. The course taken by no. 7 forms the hypotenuse 

 of a right-angled triangle, of which the other sides are formed by (a) the distance (100 m.) from the 

 end of the Parador wall A to the line-of-sight formed by the piers of the two bridges (direction of 

 wind), and (b) the distance along the direction of the wind from the starting-point C to a point oppo- 

 site the Parador wall A. This distance is almost the same as that between the two walls A and B. 

 Measurement on paper of this triangle gives the angle of drift from the down-wind direction taken by 

 no. 7 as 34 (Text-fig. 8); for no. 10 the angle was 42 . 



Plotting the result of the experiment on specimen 4 in the same way, I find that it drifted about 

 180 m. in 9 min., a speed of 20 m. a minute or 0-75 knots in a breeze of velocity rather less than 

 19 knots over the end of a flood-tide that can be ignored, f Specimens 6 and 7 made similar speeds, 

 and specimens 2 and 5 were driven rather faster. These last four experiments were made under 

 slightly different conditions. In Table 9 I suggest a comparison between boat performances and 

 Physalia in 10-15-knot wind (see Text-fig. 9). 



* The protozooid faces up-wind, the pore down-wind (see Text-fig. 5). 



f Compare this with the estimate of Woodcock (1956) of 15-6 m. per minute in a breeze of 13 knots for a specimen of 

 17 cm. in length. 



3-2 



