222 Mr. W. R. Birt on the Cyclones 



than five hundved and seventeen thousand; and this great 

 number is small compared with the immense number that 

 must have perished in the aggregate number of cyclones that 

 have occurred ; consequently any addition to our knowledge 

 of these destructive meteors cannot fail of being interesting 

 to our readers, as well as to navigators and seamen. 



Antecedents of Powerful Cyclones. 



The importance of a knowledge of the antecedents of 

 cyclones, especially of such as are attended with great loss 

 of life, cannot be overrated. For an increase of our know- 

 ledge in this respect we are indebted to J. Eliot, Esq., M.A., 

 Meteorological Reporter to the Government of Bengal, in his 

 Report on the Madras Cyclone of May 1877. In order 

 clearly to connect the antecedents with the cyclones it is 

 necessary to take into consideration the two transition- 

 periods : — one, in April or near the latter end of April, between 

 the breaking-up of the north-east monsoon and the establish- 

 ment of the south-west ; the other, in October, succeeding 

 the south-west monsoon and ushering in the north-east 

 monsoon. Mr. Eliot finds that cyclones or revolving storms 

 of great extent and intensity occur only at these periods, 

 while cyclonic disturbances of small intensity are of frequent 

 occurrence during the rainy season or south-west monsoon. 

 During the continuance of each monsoon certain meteo- 

 rological relations are established over the Bay and the ad- 

 joining land-areas; and when a change from one monsoon 

 to the other occurs — in other words, when a transition-period 

 sets in, it implies a complete reversal as regards the preceding 

 monsoon of baric gradient, wind-direction, and other atmo- 

 spheric conditions over the large area including the Arabian 

 Sea, the Bay of Bengal, India, and Further India. 



These reversals of meteorological elements occurring gra- 

 dually tend to an approximate equality and uniformity of baro- 

 metric pressure over a large area. This approximate uni- 

 formity of pressure over and round the coast of the Bay of 

 Bengal, accompanied by light variable winds and calms over 

 a considerable portion of the Bay with little or no rainfall, 

 Mr. Eliot regards as invariable antecedents of a powerful 

 cyclone. 



Connexion of Rainfall with Cyclones. 



In consequence of the small rainfall about the time of the 

 commencements of the transition-periods, the amount of 

 aqueous vapour in the atmosphere accumulates and gives rise 

 to some peculiar sky-effects. When the point of saturation 

 is attained, precipitation follows, and a heavy rainfall is con- 



