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



479 



than that at Port Blair, equal to a gradient of 1 inch in 1034 miles, while 

 that of the 1 J. C. Botelbhoe ' showed a gradient of 1 inch in 700 miles. 



Around the north and west coasts of the bay the differences of pressure 

 and its changes were inconsiderable. On the 23rd there was a slightly 

 higher pressure (0*05 in.) in the N.E. corner, which difference remained 

 unaltered on the two following days. In Ceylon also the pressure was 

 0"03 greater than at Madras ; at the same time there was a general rise of 

 the barometer, small in amount, over Bengal and the northern and western 

 coasts. On the 27th there was a general fall, and the pressures were 

 nearly equalized. 



Coincident with these changes were those of the winds. For many days 

 previously to the 24th * light south-easterly winds prevailed on the west 

 coasts of the bay, while in Bengal the wind was variable, with a predomi- 

 nance of easterly components. To the south, between the Equator and 

 N. lat. 5°, a squally damp W.N.W. wind blew continuously, having pre- 

 vailed at least from the 1 1th of October. On the 27th it became W.S.W., 

 drawing round towards the area of depression. With the barometric rise 

 on the 24th and 25th a N.E. wind set in in Bengal and down the western 

 half of the bay displacing the S.E. wind, which, however, continued to be 

 felt in the immediate neighbourhood of the Nicobars. The cyclone vortex 

 was formed by the indraught of these three currents to the preexisting area 

 of barometric depression. 



The storm chart of the Bay of Bengal drawn up by Mr. Piddington shows 

 that the majority of the cyclones, the tracks of which are there laid down, 

 proceed from a line running from south to north by the Nicobars, Anda- 

 mans, and the islands of the Arracan coast, following the westward side 

 of the mountain-axis, which, in part submarine, is a prolongation of that of 

 the Sunda Islands. Of these storms, several appear to have originated 

 in the neighbourhood of the Andamans ; but none of them have been 

 traced back to a sufficiently early period to admit of a comparison of 

 the circumstances of their origin with those of the storm now under dis- 

 cussion. The data for the great Calcutta cyclone of October 1864 dis- 

 cussed by Colonel Gastrell and myself in the report published by the 

 Bengal Government, were insufficient for a satisfactory determination of the 

 conditions under which it originated, but they offer several points of simi- 

 larity to those detailed in the preceding pages. 



The two storms agree approximately in their place of origin, in their 

 course up the bay and over Bengal, and in their termination ; and as re- 

 gards period, both occurred at the close of the S.W. monsoon. The chief 

 noticeable differences are, that the cyclone of 1864 originated about N. 

 lat. 10°, and therefore 3° or 4° further north, and on the morning of the 

 2nd of October, or nearly a month earlier. Previously to this date, for at 

 least five days, the wind in Ceylon had been from the westf or W.S.W., 

 * I have tabulated them for the six days previous. 



t We had no data to show how far to the south or south-east this direction prevailed, 



