1873.] H. F. Blanfovd— On the Climate of Bengal. 185 



those more extensive and destructive storms, that originate over the Bay of 

 Bengal, and are most frequent at the changes of the monsoons. These lat- 

 ter have received the distinctive name of Cyclones ; and the name is perhaps 

 as good as any other, since in them a vorticose motion of the wind is a 

 strongly marked character, and one of great practical importance ; but it is 

 by no means a character peculiar to these storms, since it may frequently 

 be observed in a slight degree in the ordinary North-Westers, and Tornados 

 which are apparently merely a severe form of the North- Wester, and differ 

 from a typical cyclone only in their originating over the land, in their inferior 

 size and shorter duration. It may be indeed that the direction of their 

 circulation is not so constant as in the greater storms, but existing evidence 

 is insufficient to settle this point. The dust-storms of the Upper Provinces 

 also, have been shewn by Dr. Baddeley to consist of one principal and numer- 

 ous minor vortices, exactly like the larger storms of oceanic origin. The pres- 

 sure of the wind in Tornados and even in ordinary North- Westers is some- 

 times comparable with that of cyclones, and, within a limited area, the former 

 are not less destructive. There is an important difference in the character 

 of the surface wind in these two forms of land storms. In the North- Wes- 

 ter the violent wind usually precedes the storm, blowing outwards, and being 

 in fact a descending current brought down by the friction of the falling 

 rain. The centripetal currents which feed the storm are not felt at the 

 ground surface, though they may frequently be traced in the motions of the 

 lower clouds. In the Tornado, on the other hand, as in the true cyclone, the 

 violent surface winds are centripetal and vorticose. 



The Cyclones felt in Bengal begin, in all cases, over the Bay ; and the 

 more violent and extensive storms, which alone reach the land, probably require 

 many days to form before they move forward from their place of origin. 

 Some of the most destructive that have passed over Bengal, have proceeded 

 from the neighbourhood of the Andaman and Nicobar Islands. Their rela- 

 tive frequency in the different months of the year is shewn in the following 

 table, which includes storms of all parts of the Bay, and those that have 

 been felt on all parts of its coasts, Bengal included. 



January, 2 May, 17 September, 3 



February, ... June, 4 October, 20 



March, 1 July, 2 November, 11 



April, 5 August, 2 December, 3 



Of these seventy-three storms, twenty-three have been felt in Bengal 

 or on its coasts, and all between the months of April and November inclu- 

 sive. Their course is usually North across the Gangetic Delta, North West 

 from the Orissa coast. The motion of the wind is in an involute spiral, 

 revolving in a direction opposite to that of the hands of a clock, as in all 

 cyclonic storms in the Northern Hemisphere. The greatest pressure of the 



