216 spolia zeylanica. 



Drift-bottle Experiments. 



Southwell and Kerkham have rendered valuable service by 

 their drift-bottle experiments. Previous workers had realized the 

 possible importance of oceanic currents in relation to the dispersal 

 of the pelagic stages of the oyster, and Herdman recommended the 

 use of drift bottles in order to extend our knowledge of the currents 

 in the neighbourhood of the Gulf of Mannar. I believe I am right 

 in saymg that Hornell first started the drift-bottle experiments, 

 but the results have never been published. Hence the result of 

 Southwell's and Kerkham's drift-bottle experiments in the Gulf of 

 Mannar are the first which have been published, and they disclose 

 an interesting state of affairs. 



In the north-east monsoon an oceanic current makes a northward 

 sweep up the west coast of Ceylon, becomes deflected at Tallaivillu 

 Point, and from westwards towards Cape Comorin. Hence this 

 current does not reach the pearl banks. On the banks there is a 

 steady wind from the north which sets up a surface drift, which when 

 the monsoon is strongest, from November to January, is sufficient to 

 carry floating objects from the pearl banks as far as the westward 

 oceanic current flowing towards Cape Comorin. 



The drift experiments have revealed two phases during the south- 

 west monsoon : (a) When the monsoon is strong the water at the 

 head of the Gulf becomes piled up, and an easterly oceanic current 

 striking Cape Comorin is partly deflected across the mouth of the 

 Gulf of Mannar ; when it reaches Tallaivillu Point the current takes 

 a southerly course. The stronger the monsoon the higher up the 

 Gulf is this current forced, and may reach as far as north as the 

 Tuticorin banks, whence it is deflected eastwards over the Ceylon 

 pearl banks. 



(6) When the monsoon is weak quite a different state of affairs 

 exists, though the real significance of the conditions during this 

 phase is not rendered quite clear by Southwell and Kerkham. 

 They say that " during a weak or moderate monsoon this current 

 never penetrates the Gulf at all." 



In view of this statement, it is difficult to explain the northerly 

 flow along the Indian side of the Gulf of Mannar, which they show 

 in chart B, and to which they refer in the text as finding an outlet 

 through the Paumben (Jhannel. 



Two alternative explanations of the conditions prevailing in a 

 weak south-west monsoon suggest themselves to the writer. The 

 first is, that the oceanic current, after striking Cape ( /omorm, is not 

 deflected eastwards across the mouth ol' the Gulf, ))ut owing to the 

 lightness of the wind is able to take a juuch more southerly course, 

 and thus may miss the Ceylon coast altogether, or only strike it at 

 the southern extremity. In such a case a large triangular area lying 

 to the north of this current is unaffected by it. In this area a light 

 surface drift is set up by the south-westerly wind — a drift which is 



