TRANSACTIONS OF THE SECTIONS. 23 



are shipmasters and others who put little faith in the theory of revolving storms. 

 But a careful investigation of all the ;jreat storms which have occurred iu the In- 

 dian Ocean, south of the equator, during- the last eighteen years, has amply con- 

 firmed the truth of the theory iu the main. On the other hand, some corrections 

 and modihcations are required. 



These rotatory storms, which are confined to the months of November to May in- 

 chisive, originate between the parallels of 6° to 14° S., and travel to the W.S'W., 

 and afterwards, but not always, to the southward and S.E. ; the wind invariably 

 moving round a central space (which is usually characterised by a calm) from 

 left to right, or with the hands of a watch ; while the stonn, which has a diameter 

 of 1 to 1500 miles, moves onwards at the rate of 1 to 20 miles, but more frequently 

 4 to 7 miles an hour, for a period varying from a few hours to ten davs, attended 

 with torrents of rain, and in its northern half often with lightning. 



It would appear that when they were tirst made a subject of investigation, 

 attention was chiefly directed to what took place within the storm, all the infor- 

 mation regarding it having been derived fi-om a few vessels which had been involved 

 in it; while little notice was taken of the state of the prevailing winds at a distance, 

 or of the possible connexion between them and the origin and progress of the storm. 

 Hence some waiters appear to have regarded them as detached disks of air, put 

 and sustained in motion by electricity, magnetism, earthquakes, or some other 

 mysterious agency. 



One of the lirst results of the extended system of observation adopted at Mauritius 

 was to show, what had been surmised by Dr. Thom, that these revolving storms 

 are invariably generated between the N.W. monsoon and the S.E. trade-wind, and 

 that to all appearance their rise and progress are intimatel}' connected with those 

 two opposing winds. The fact that they occur only during the monsoon months 

 in itself favours the supposition of a connexion between the two phenomena. 



Observation has shown that the monsoon extends farther south on the western 

 than on the eastern side of the Ocean, its southern limits often stretching obliquely 

 from Tamatave, in Madagascar, on the west, to Sumatra on the east. To the south 

 of the N.W. monsoon the S.E. trade-wind prevails. Between the two winds there 

 is a space of calms, or light variables. During hot sidtry weather evaporation 

 must take place rapidly, especially in the trade-wind region. The vapour is carried 

 by the two winds towards the space w^hich separates them, and is accumulated 

 there until the air becomes saturated. There may at the same time be an ascend- 

 ing column of air and vapour, which would further promote condensation. Heavy 

 rain sets in, the barometer falls, and the two oppositely directed wnnds flow to- 

 wards the locality of diminished pi-essm-e, bringing with them more vapour, which 

 is also speedily converted into rain, the barometer falling lower. 



As the vapour is chiefly supplied by the S.E. trade-^\dud, and its precipitation in 

 the trade-wmd region is followed by a decrease of barometric pressure there, the 

 movement of the area of diminished pressure is towards the south, across the trade- 

 wind region, the N.W. monsoon, and the N.E. trade-wind to the north of it, where 

 tlie barometer is high, pressing to the southward to restore the equilibrium, and 

 the monsoon, as it were, eating into the trade-wind as the aqueous precipitation 

 proceeds. In this way the monsoon sometimes- advances along its whole extent in 

 longitude to the tropic of Capricorn, or even beyond it, until the trade-wind alto- 

 gether disappears, or is found only far to the south. When the vapour has been 

 precipitated the trade-wind gradually returns, the monsoon receding before it to 

 the northward, imtil the two winds again attain their normal positions. After a 

 lapse of some time, during which another accumulation of vapour takes place, 

 heavy rains a^-ain commence on the equatorial borders of the trade-wind, and the 

 monsoon again advances to the southward. The two winds thus oscillate back- 

 wards and forwards during the summer months, and it is on these occasions, when 

 the monsoon is advancing to the southward, that the tropical revolving storms 

 occur, the south-west and west sides of the storm being apparently fed by the 

 trade-wind, and its north-east and east sides bj' the monsoon. 



Instances of the advancement of the monsoon to the southward, as from the 

 l.Sth-to the ISth of February, 1800, the 16th to the 20th of January, 1861, the 1st 

 to the ISth of February, 1861, and the 16th to the 24th of February, I860, were given 

 in a series of charts showing the directions of the wind at noon on each day. 



