214 spolia zeylanica. 



Currents. 



According to Herdman and Hornell, the surroundings of the 

 oysters are mfluenced in some degree by the presence of bottom 

 currents, and they were of opinion that one of the causes of mortality 

 among young oysters was the silting of sand over the oysters through 

 the agency of these bottom currents. In fact, they regarded the 

 shifting sand as a more serious cause of disaster than even the 

 depredations of voracious fish. 



In their discussion of the bottom currents, 8outhweli and Kerk- 

 ham assumed that the danger of a bottom current to a bed of oysters 

 is that the oysters will be swept away, and apparently on those 

 grounds they appear to ignore the more probable explanation 

 suggested by Herdman, that any movement of the bottom layers of 

 water is dangerous, in that the sand becomes silted and buries the 

 oysters. 



Southwell and Kerkhani contend that there is no longitudinal 

 motion of the bottom layers of water, and base their conclusions upon 

 three series of observations. Experiments Avere made with a piece 

 of apparatus which was too faulty to be treated seriously, and they 

 themselves admit that the results of these experiments were " of too 

 crude a nature to count for anything." Their second ground for 

 adopting this view was based upon the scanty information obtained 

 from the divers. But the main reason for their opinion is founded 

 upon the observation made by Captain Kerkham himself on the 

 numerous occasions when he descended in diving dress. It is 

 significant, however, that none of these descents were made in the 

 height of the south-west monsoon, when the banks are swept by 

 strong winds and the water is in a continual state of disturbance.* 

 Whether even in the roughest weather the -water six fathoms below 

 the surface is affected in any considerable degree by the action of 

 the Avind is a doubtful point, but that some serious disturbance of 

 the bottom layers does take place during the south-west monsoon is 

 actually admitted by Southwell and Kerkham when they state that 

 '" at such a time (late in October, before the south-west monsoon has 

 subsided) the bottom layer of water is turbid. The cause of this 

 turbidity has been microscopically examined, and has been repeatedly 

 proved to be clue to the disintegrated remains of seaweed and 

 caulerpas. The turbidity renders it almost impossible for divers 

 to see, but it subsides as the north-east monsoon begins." They 

 admit, mcM-eover, that the sandy bottom becomes ridged during the 

 south-west monsoon. This ridging is caused, they assert, by the 

 action of surface agitation, which is transmitted as a vertical and not 

 as a horizontal movement. This evidence may quite well be claimed 

 by Herdman as support for his case. It seems clear that any move- 

 ment, be it horizontal or vertical, which can produce well-defined 



* Dre.ss diving is impossible during the south-west monsoon owing to bad 

 weather, but this is the period of the year when silting will take place, if ever 



