1888.] occurred at Dacca on April 7th, 1888. 191 



sional motion ; and this probably forms an important part in the develop- 

 ment of the storm and of its well known energy. Or the north-westerly 

 current which is blowing may suddenly overcome the resistance of the 

 southerly current, and cool air from the higher regions of the atmos- 

 phere may force its way downwards to the earth's surface and cause 

 similar effects. The formation, however, of these nor'westers appears 

 to necessitate a considerable amount of air motion, and also that the air 

 currents shall be of more or less opposite nature. 



This, however, does not exhaust the possibilities of storms in India, 

 and, in many cases, dust-storms, &c. are formed when there are apparently 

 no opposing wind systems at work as previously described. Such 

 storms usually form at considerable distances inland and over highly 

 heated and dry land surfaces, and their formation is probably due 

 to the intense heating effect of the sun's rays on an atmosphere 

 more or less laden with dust and other solid particles. The immediate 

 antecedent cause of the formation of such a storm may perhaps 

 be found in an action which may be best described in the words 

 of Sir George Airy, who states, " The atmosphere is a viscous gas, and it 

 is only on this assumption that cyclonic phenomena and the phenomena 

 of all rotatory storms in the hotter parts of the earth can be explained, 

 and that in such storms there is a mass of hot air which, from the vis- 

 cosity of its structure, is not able to rise up for a long time until at last 

 it rises up with a burst." It is in fact only by some such action as this 

 that many of the phenomena of hot weather storms in India can be ex- 

 plained. If we admit that, over a considerable tract of land, owing to 

 the heating effect of the sun and the viscosity of the air, there is a 

 decided increase of pressure, which, after accumulating for a time, is 

 suddenly relieved, and that in consequence of this relief of pressure the 

 highly heated air suddenly ascends, then the uprush will, directly the 

 ascensional motion commences, assume a spiral movement, and there 

 will be formed in the northern hemisphere a wind rotation probably 

 against the hands of a watch, similar in fact to that sometimes observed 

 in dust-storms and usually in tornadoes in the northern hemisphere. 

 Though these storms can be understood so far, their enormous energy 

 has not been well accounted for, though many theories and suggestions 

 have been put forward. 



Theories such as have just been very briefly and incompletely 

 described are advanced to account for the classes of storms which arc 

 frequently met with in India during the hot season of the year, but, from 

 time to time, perhaps once in five or ten years, the conditions which 

 usually are only followed by ordinary nor'westers or dust-storms, but 

 which are more or less violent in their nature, give birth to a whirlwind 

 or tornado of extreme energy and destructive force. In the case of the 



