434 A Second Memoir with reference to [No. 100. 



subject of the uncertainty which besets us, either from the state of the 

 weather, or other causes, rendering it impossible to ascertain exactly 

 the ship's position, or the true direction of the wind. I may add here, 

 that we should further consider, that in a circle of 320 miles circum- 

 ference, or about 100 in diameter, each point of the compass includes 

 an area of ten miles, which may be considered as a trifling error in 

 a vessel's reckoning in bad weather, so that if the wind be marked a 

 point wrong, and the vessel's situation also be only ten miles wrong, 

 and both these errors the same way, we may thus find a considerable 

 discrepancy appearing where none really existed. I mention this for 

 the information of those, who might perhaps look for more mathe- 

 matical exactness than our data will allow, and because I am desirous 

 of shewing how important correct observations are to us in tracing 

 out, step by step, the truths or probabilities which we deduce from them. 



It will be more convenient to consider this storm under the follow- 

 ing heads. 



I. — The formation of the vortex, or circle of the tempest, and 

 evidence for its form. 



II Its size. 



III. — Its rate of progression. 



I. — The formation of the vortex, or circle of the tempest, and 

 evidence for its form. 



We find that on the 12th at noon, (Diagram No. I.,) the Duke of 

 Bedford, Balguerie, Sumatra, and Cashmere Merchant, were all on 

 the eastern side of the Bay, between Lat. 11° 30' and 17° 00' N. 

 and long. 90° 00' E. and 92 3 30' E. Of these, we find the Sumatra 

 with the wind at ESE., and the Balguerie with it at, WNW. 

 both with a strong breeze, and from this last being opposed to the 

 usual monsoon current of that season (which is from the northeast- 

 ward) we may suppose a vortex to be commencing. The monsoon 

 seems to have been blowing steadily all across the bay, at least from 

 the lat. of 14°, for we find on this day the Arethusa two degrees to 

 the eastward of Coringa, and the Ganges Steamer in the middle 

 of the Bay, in about 18° 30' N., all with the winds between East 

 and ENE. We may then conclude that the disturbing force, what- 

 ever this was, had only began to operate below the latitude of 13°, 

 and near the Andaman Islands, and that its centre, deduced from the 

 positions of the Balguerie and Sumatra, was about where I have placed 



