234 CEYLON MARINE BIOLOGICAL REPORTS. 



It is interesting to further find tjiat Steuart in his " Notes on Ceylon " (1843) points out that 

 "ships leaving Bombay in the height of tho north-east monsoon, to load cotton at Tuticorin, after 

 passing Capo Coniorin find tho soutliern current so strong as to induce them to stretcli across the Gulf 

 of Mannar, and beat up the western coast of Cej'lon until tliey can steer for their destined port, and 

 this they accomplish against what is called in Ceylon ' a long shore wind.' " 



This old-time observation fully amplifies our statement of the prevalent currents, and falls in line 

 absolutely with our own observations. 



To summarize, then, on the west coast of Ceylon the surface disturbance falls into two sharply 

 defuied sets, each set being strictly limited to a particular area, as under : — 



(1) North of Tallaivillu Point (which comprises the pearl bank area under lease) the conditions 

 of entire protection from oceanic currents and from the north-east monsoon exist on the insliore areas. 

 Such disturbances as there are are purely local, varying, and intermittent, and are entirely due to the 

 \\'ind, which is invariably from the north over the plateau. Tliis surface drift running south finds a ready 

 exit in Portugal and Dutch Bay, and the changes taldng place in the latter backwater may in some measure 

 be due to the presence of this surface drift. 



Further out at sea, and to the west of the plateau, northerly winds stiU prevail, and consequently 

 tliere is a defuiite surface drift, which runs south for some distance. Eventually this drift joins the 

 current running nortli (in the opposite direction) along the West Ceylon coast, and from about Tallaivillu 

 Point rmis west. Some bottles liberated about this vicinity have been recovered on the East Indian 

 coast. The majoritj', however, were lost. Li some cases a few rounded Cape Comorin, and were 

 recovered on the West Indian coast as far north as CaHcut, but the main current passes west into the 

 Indian Ocean, and the bottles were consequently lost. One alone was returned from the Maldive Islands. 



(2) South of Tallaivillu Point a definite current runs from the south of the Island in a northerly 

 direction at an average rate of 0-3 knot per hour, and in the face of the north-east monsoon. This 

 current is deflected about Tallaivillu Point, and from there takes a westerly course towards the Indian 

 coast, and then, turning south, rounds Cape Comorin, and has been traced north along the west coast 

 of Lidia as far as Calicut. But the main current runs west into the Indian Ocean. 



It is cei'tain that the secluded and sheltered position of the pearl bank area under lease as distinct 

 from the pearl bank area still under Government control, and which latter hes south of Tallaivillu Point, 

 owes its larger degree of productivity to the incidence of its protected position. It is also evident, 

 a priori, that such surface disturbance as does exist there is not sufficient to affect oj'sters whose natural 

 habitat is in from 5 to 10 fathoms of water. 



It might be argued that on a shallow water plateau such as exists on the west of the Island no 

 surface current or drift could exist at all which did not affect the bottom on which the oyster lives. 

 There are certainly movements of the water on the bottom, otherwise stagnation would result. But 

 these movements are undulatory. They are vertical, and not horizontal. 



The total efiEect of the surface movements on the area imder lease with regard to the oyster is nil. 

 They certainly are not harmful. The relative productivities of the areas (a) under lease and (h) those 

 still under Government control has a deeper origin. But before attempting to discuss the matter 

 further, we must first understand the nature of the surface currents during the south-west monsoon, for 

 these currents in toto have a very real relation with the pearl oyster. 



II. — SuBFACE Currents during the South-west Monsoon. 



We have already pointed out in Part III. Report of this series that the south-west monsoon 

 on striking Cape Comorin is deflected in such a way that it runs approximatelj' east and reaches the 

 Ceylon coast about Tallaivillu Point, and then runs south. 



It will be noted that the current, therefore, runs exactly opposite to the one prevalent in the north- 

 east monsoon, and not onty has it a definite course, but a more definite origin. This oceanic current, 

 running as it does in its easterly course from Cape Comorin (and which has been felt as far north as 

 Tuticorin), plays a very prominent and important part in the determination of the currents prevalent in 



