1884.] of the Bay of Bengal in 1883. 185 



The remaining conditions appear to be necessary in order that the 

 rainfall may occur in such a manner as to give rise to and produce an 

 atmospheric whirl. It is evident that if rainfall tends to set up rotatory 

 motion in the air, it is absolutely necessary for rotatory motion on the 

 large scale that there should not be several separate centres of rainfall 

 and disturbance, each producing its own rotatory or cyclonic action, and 

 therefore interfering with the others. It is essential that the rainfall 

 should be localized and concentrated, that it should continue for some 

 time over a comparatively small area, and be confined to that area. The 

 more perfectly this is realized, and the longer this continues, the greater 

 will be the accumulated disturbance. In order that the rainfall may 

 occur over the same area for such a considerable period as to permit of 

 the continuous accumulation of action, it is evident that ascensional 

 motion should mainly occur there, and hence that, previously, there should 

 be little horizontal motion of the air, and therefore very slight differences 

 of pressure at the sea level. The necessity for the further conditions 

 is hence also evident. 



It will be seen that these conditions were fulfilled in the case of 

 both storms, more completely (as might have been anticipated) in the 

 case of the second storm, when the south-west monsoon current was 

 weaker than it was at the time of the first storm. The history and 

 discussion thus fully bear out the existence of the conditions immediately 

 antecedent to the two storms which the condensation theory asserts to be 

 necessary for the initiation and generation of a cyclonic storm in the 

 Bay. 



The preceding remarks hence indicate that the energy given out 

 during the process of aqueous vapour condensation on the large scale is 

 the motive power of cyclones, and that the rainfall must be localized and 

 concentrated over a considerable area, for a period of one or more 

 days, in order to produce the continuous and rapid accumulation of 

 energy which characterizes a large cyclonic disturbance. Experience has 

 also shown that the conditions which the condensation theory suggests 

 as being essential for the occurrence of continuous and prolonged local 

 rainfall over a portion of the Bay are exactly those which are present 

 before and during all cyclonic storms in the Bay of Bengal, and that they 

 are more fully marked before the occurrence of the larger than of the 

 smaller cyclones of the Bay. It is, moreover, these antecedent conditions 

 which form the only test or indication of the possible or probable early 

 formation of cyclones in the Bay, and which are utilized in the preparation 

 of the daily weather Reports issued by the India and Bengal Meteorolo- 

 gical Departments. 



