1869.] Mr. H. F. Blanford on the Origin of a Cyclone. 481 



more elevated temperature it would impart to the atmosphere, would give 

 rise to a diminution of barometric pressure, small at first, but becoming 

 more marked with the continued operation of the producing cause, so that 

 after several days it might become capable of causing that extensive in- 

 draught of air which appears to be the immediate antecedent of the forma- 

 tion of a cyclone vortex. 



According to Horsburgh, from April to the early part or middle of 

 October, a current generally sets to the north or north-east all over the 

 bay in the open sea, but governed in its direction and strength by the pre- 

 vailing winds. " In the eastern side of the bay, and about the entrance to 

 the Malacca Strait more particularly, it sometimes sets to the southward. 

 The current begins to set along the coast of Coromandel to the southward 

 in October, sometimes about the middle of the month. Near the end of 

 this month, or early in November, it begins to run very strong to the 

 southward." He adds, however, that the period of the currents or mon^ 

 soons changing in the Bay of Bengal is not always the same. These changes 

 happen in some years nearly a month sooner or later than in others. 



From this and the remainder of the description, it is to be gathered that 

 the set of the current changes with the monsoon, and that in general its 

 direction and strength are governed by the prevailing winds. Now it is 

 well known that at the change from the south-west to the north-east mon- 

 soou, the latter is first felt down the western part of the bay, and that at 

 this season ships bound to Calcutta from the southward keep up the east 

 of the bay, with a view to catching a favourable wind. It might be ex- 

 pected, therefore, that there would be a tendency to northerly currents in 

 the east of the bay, and to southerly currents in the west ; and, so far as 

 can be gathered from Horsburgh's description, such indeed appears to be 

 the case ; but facts are at present wanting to establish it, and also the 

 existence of those differences of water-temperature which would seem to 

 be the necessary consequence. 



It would appear from Horsburgh's description that, at the commence- 

 ment of the south-east monsoon in May (the minor of the two cyclone 

 seasons), the tendency of the currents is the opposite of the above, that 

 from January to June a northerly current sets strongly up the Coromandel 

 coast, and that from April there is a current to the north or north-east 

 all over the bay, but on the eastern side of the bay, and particularly the 

 entrance to Malacca Strait, it sometimes sets to the south. Now from 

 April to August the sun is vertical over the northern part of the bay ; but 

 no data are available to show how the temperature of the currents is affected 

 by this change in its declination, and I am unable therefore to ascertain 

 how far the supposed condition of a higher temperature in the currents of 

 the east of the bay holds good in the case of those barometric depressions 

 which determine the cyclones of the beginning of the south-west monsoon, 

 and which Piddington's chart shows to originate in that region. I may, 

 however, remark that, as far as I can gather from the recorded cyclones I 



