478 Mr. H. F. Blanford on the Origin of a Cyclone. [June 17, 



stratus and scud moving rapidly from the westward. To the S.E., between 

 Sumatra and the Malacca peninsula, the 'T. A. Gibb' encountered cloudy 

 weather with occasional squalls and variable or southerly winds. Her 

 barometer has not been compared with the Calcutta standards*, but, as 

 far as can be judged by a comparison of its reading at the Sandheads with 

 that of the Saugor Island instrument at a later date, the actual barometric 

 pressure on the 2 7th, in lat. 2° 18' N., long. 101° 56' E., would seem to 

 be about 29'8, or nearly the same as that recorded by the ' Astracan' on 

 the Equator, 12 degrees to the westward, on the same date. 



In and around the area of maximum depression a cyclone had already 

 formed. Its centre was probably somewhere between the Andamans and 

 Nicobars, as indicated by the wind-directions of the ( J. C. Botelbhoe,' the 

 ' Timoor Shah ' and ' Comorin,' and the ' Gauntlet and that its force was 

 considerable may be inferred from the fact that the 1 Eerose Shah/ bound 

 from Carical to Penang, was dismasted on the 2 7th and driven on a bank 

 near the Little Andaman, known as the South Brother. The four ships above 

 mentioned experienced hard squalls and heavy rain, and the * Timoor Shah 3 

 describes the wind as blowing a hard gale in the after part of the day. 



During the five days under notice there appears to have been little 

 change in the prevalent temperatures. A general fall of from 1 to 2 de- 

 grees is the utmost shown by the temperature Table given on a previous 

 page. The decrease in the humidity is more marked at all the land stations, 

 but especially at Patna, owing probably to the increasing prevalence of a 

 northerly or north-westerly wind. It is much to be regretted that, owing 

 to Mr. Barnes's departure from Ceylon, the valuable meteorological record 

 which that gentleman used to keep, and an extract from which he was 

 able to furnish for the discussion of the storm in 1864, is no longer avail- 

 able ; and I am unable to ascertain whether the humidity of the atmo- 

 sphere in Ceylon was as high as before the cyclone of 1864. 



The principal facts exhibited in the foregoing description may be summed 

 up as follows : — 



For at least four days previously to the formation of the cyclone vortex 

 the barometric pressure to westward of the Nicobars and the northern ex- 

 tremity of Sumatra was lower than elsewhere in or around the bay. It 

 was also lower (on the 24th of October certainly, and probably on the 

 previous day also) than on the open sea to the southward. The depres- 

 sion was gradually intensified up to the 27th, when it began to blow a 

 hurricane on the northern limit of this area. It then amounted to — 0'4 

 of the pressure in Bengal, —0*36 of that in Madras, and —0*22 of that on 

 the Equator. It would appear, however, that over the greater part of the 

 bay the pressure was nearly equable, and that the depression was local and 

 bounded by a much higher barometric gradient than would be indicated 

 by the figures above given. Thus the ' Gauntlet' reading was 0'29 less 



* It v? as sent to me for comparison, but was injured in the carriage, so that no ccm- 

 arison could be made, and the tube had to be replaced. 



