1884.] of the Bay of Bengal in 1883. 183 



cnlations. Other causes of the origin of cyclones have been assigned, 

 as, for instance, differences of pressure, friction between parallel winds 

 blowing from opposite directions, &c., but the slightest consideration seems 

 to show that none of these is sufficient to account for the enormous and 

 continuous transfer of energy that occurs during the prolonged existence 

 of a large cyclone. The strongest argument against these theories, in the 

 case of cyclones of the Bay of Bengal, is, that experience has established 

 that the larger the cyclone, the smaller are the antecedent differences of 

 pressure, and the feebler are the winds blowing from opposite directions, 

 immediately before the formation of the cyclonic vortex. 



The following statements based on the preceding remarks hence give 

 the answer to the first part of the required explanation. When water is 

 converted into aqueous vapour on the large scale at the earth's surface, 

 thermal energy, derived from the sun, performs the work of evaporation, 

 and is hence transformed. The aqueous vapour thus produced possesses an 

 equivalent amount of energy, the greater part, if not the whole, of which 

 it retains, so long as it continues in the vaporous condition. When it is 

 reconverted into water, or condensed as rain, this portion of its total 

 energy is given out, and transferred to the air. The modus o'perandi 

 of this transfer is a matter of no importance in the present enquiry. 

 Also, in all cases when the rainfall is heavy, and prolonged for a con- 

 siderable time, the energy is given out at a much more rapid rate 

 than that at which it was absorbed during the process of evaporation. 

 Hence heavy and prolonged rainfall may give rise to a powerful, persis- 

 tent, and continuously accumulating disturbance on the adjacent atmos- 

 pbere, and, therefore, produce violent and extensive air motion. In virtue 

 of the constitution of the atmosphere, the motion will be rotatory. Pro- 

 longed heavy local rainfall is hence an adequate and sufficient cause. It 

 is, moreover, the only known cause which is equal or similar in amount 

 to the effect, and hence there are strong reasons for assuming that it is the 

 motive power which produces the peculiar motion of the atmosphere called 

 cyclonic circulation on the large scale. It is, in fact, the most powerful 

 disturbing action to which the air is subject, and the consequent motion 

 of the air is, when the rainfall and consequent disturbance are excessive, 

 the most violent in its character with which we are acquainted. 



The history of the two cyclones has shewn most fully that heavy 

 rainfall over the area of cyclonic motion or disturbance was a charac- 

 teristic feature, and that in this respect they confirm previous experience. 

 Hence the source of the energy of these two cyclones was almost cer- 

 tainly that which we liave indicated in the previous statement, that is^ 

 the latent heat energy of the aqueous vapour derived previously from 

 the sun, and transferred to the atmosphere during the process of con- 

 densation. 



