CURRENTS. . 237 



penetration the surface movements in the gulf are purely due to wind, which near the Lidian coast 

 blows from the west, whilst further out at sea the wind becomes soutli-west. 



It is important to note that during the south-west conditions a strong westerly wind is continually 

 blowing with some force over Southern India, and is persistently recorded at Tuticorin. The oceanic 

 current is thus assisted considerably in its easterly course by the prevailing wind. 



We thus see that the surface currents during the south-west monsoon have two phases, and it 

 will be weU to here summarize them : — 



(1) A hght or moderate monsoon, when the currents rim northerly towards the Paumben Pass, 

 both along the Indian coast and along the Ceylon coast south of Tallaivillu Point (only), where the water 

 finds an exit. 



(2) A phase of strong monsoon, when the oceanic current penetrates the gulf, and the water at the 

 head of the gulf consequently becomes piled up as a cushion. The oceanic current, aided by the westerly 

 wuid which prevails over Southern India, takes an easterly course, and eventually reaches the Ceylon 

 coast about Tallaivillu Point. It is here joined by an overflow of water from the head of the gulf and 

 then runs south on the Ceylon coast. This penetration of the oceanic current into the guK destroys the 

 northerly set towards the head of the guU, for the forces which normally produce this northerly current 

 have been expended in pushing the oceanic current north. The drift over the volume of water north of 

 where the oceanic current penetrates is solely due to the prevailing ^inds ; whilst at Paumben Pass and 

 Tallaivillu Point there are exits for the water therefrom, -which, being above the normal level owing 

 to the penetration of the oceanic current, is under pressure. 



In the table given at the end of this paper an interesting transition between the two phases of the 

 south-west monsoon current is shown. (Chart B.) 



On July 25 strong monsoon prevailed and the oceanic current ran east, as indicated on Chart A. 



On July 28, 20 drift bottles were liberated just off Tallaivillu Point. These travelled on the head 

 of the water north of the penetration of the oceanic current and were blown ashore at Mannar, where 

 18 were recovered. 



This fact indicates that the water towards the head of the gulf dm-ing a strong monsoon is dead, 

 and that the drains or outlets to this head of water at Paumben and TaUaiviUu merely relieve the 

 pressure without producing currents. The surface movement at such a time is merely due to the wind, 

 as was shown by the bottles going ashore at Mannar. 



A week later, as the lull in the monsoon continued, the head of water had had time to drain 

 away, and a northerly set of the current was re-established. It was found that of 20 bottles liberated 

 at the same station, 17 were recovered at Paumben. These facts are most instructive, and clearly 

 corroborate the ideas regarding the currents during the south-west monsoon set forth in this paper. 



During our investigations this year we found that the monsoon went through each phase twice 

 and then died a%\ay. We attach no importance to these numbers at all, believing as we do tliat tlie phases 

 named alternate with each other irregularly year by year. 



Two things now become clear. On the pearl banks under lease, i.e., those north of Tallaivillu 

 Point, there are no true currents during this monsoon. There is a surface drift trending to the beach 

 on uishore areas and due to the strong prevailing \vind, but the banks are completely out of Une with, the 

 chrection of cither phase of the surface south-west current. On the banks south of Tallaivillu Point, 

 and stiU under Government control, the current runs south and in the teeth of the monsoon when 

 the monsoon is strong ; whilst when the monsoon is weak or moderate the current merely becomes a 

 surface drift, due, as on the banks under lease, to the wind, and invariably running north towards 

 the shore. 



In our report to the Ceylon Government on the inspection of their banks, i.e., those south of 

 Tallaivillu Point (Part III., " Ceylon Marine Biological Reports"), we stated that the banks under lease 

 owed their productivity to the fact that they were protected, and that owing to the deflected current above 

 described being furtlier acted upon by the south-west wind and sea, exotic spat drifted on to our banks, 



