628 
ME. H. F. BLANEOED ON THE WINDS OE NOETIIEEN INDIA. 
winds are light and variable over the open sea, is a condition favourable to the forma- 
tion of these storms, and that a second condition is a high or moderately high tempera- 
ture. The consequence of this collocation will be the production and ascent of a large 
quantity of vapour, which will be condensed with the liberation of its latent heat over 
the place of its production, instead of being carried away to some distant region. If 
this state of things last for some days, the atmospheric pressure will be locally lowered, 
causing, or tending to cause, an indraught of air towards the place of minimum pressure. 
In order that a cyclone may ensue, one further condition appears to be essential. It was 
pointed out in a paper on the origin of the November cyclone, 1867, published in the 
Proceedings of the Royal Society *, that most of the storms of the Bay of Bengal origi- 
nate along a line running from south to north by the Nicobars, Andamans, and the 
islands of the Arakan coast. Some, such as those of the 5th and 10th June and the 7th 
and 8th October, 1869 ( supraYV ., V.), also a storm that occurred during the present year 
(1872) on the 30th June and 1st July f, originate about the middle of the Bay. But I do 
not know of a single case in which a cyclone has been actually formed in the north-west 
of the Bay, or under the lee of the Madras coast, although more than one case has been 
recorded in which most of the requisite conditions were fulfilled, and in which for some 
days there has seemed reason for apprehending onej. These facts, I think, find their 
explanation in another fact, surmised by Colonel Gastrell and myself as a probable 
local law in 1865 §, and verified in the case of every storm I have yet investigated 
(where sufficient evidence has been forthcoming), viz. that the formation of a cyclone is 
determined by an inrush of a saturated stormy current from the south-west or west- 
south-west. Now, under the lee of the Madras coast or in the north-west of the Bay, any 
wind coming from this direction must pass over the peninsula, a course which would drain 
it to a great extent of its vapour, while its free passage would be enormously impeded 
by friction and the irregularities of the land-surface ; but in the eastern half of the Bay 
no such impediments present themselves to its free access, and its high velocity and 
abundant vapour seem to be the determining conditions of the formation of cyclones. 
I would especially guard myself against being supposed to extend the above views, 
mutatis mutandis, to the case of the South Indian Ocean, or, indeed, any area other than 
that I am specially dealing with. Doubtless it will be found that in each region sub- 
ject to cyclones the determining conditions present local modifications; and the best 
way to arrive at a general theory of the formation of cyclonic storms will be to ascertain 
the local conditions as accurately as possible in each case by independent study, as 
Mr. Meldrum has done in the South Indian Ocean, and as I have here attempted for 
the Bay of Bengal. It will then be easy to eliminate all that is merely local, and to 
establish general laws by a comparison of the results so obtained. 
* Proceedings of the Royal Society, vol. xvii. p. 479. 
t Described by Mr. W. Gr. Whlsox in a Report to the Government of Bengal. 
+ See the Bengal Meteorological Reports for 1870, p. 115, and 1871, p. 124. Another case occurred during 
the present year on the 26th October. 
§ Report on the Calcutta Cyclone of 1864, p. 105,- &c. 
