268 



REPORT 1861. 



most slowly rotating, considering the cyclone merely with reference to its own in- 

 ternal motion of rotation, and not with reference to the difference which would be 

 perceived by a ship at sea, the latter being due to the fact that one side of a cyclone 

 moved with the trade-wind, and the opposite against it, and resulting in a general 

 translation of the entire cyclone, along with the trade-wind, to be considered sepa- 

 rately ; but the most westerly quarter of the cyclone being the slowest moving aa 

 regards its own internal rotation, it follows that the cyclone will progress along a 

 line at a tangent to this, that is to say, southwards or nearly so in the southern 

 hemisphere, and northwards or nearly so in the northern hemisphere, the direction 

 of rotation being here the reverse of what it is in the southern hemisphere. Com- 

 bining this, the proper motion of the cyclone, with the motion derived from the 

 trade-wind, both being represented by the dotted lines in the annexed figure of a 

 cyclone in the southern hemisphere, we obtain the actual motion in a direction S. W. 



An 



^ SW 



or thereabouts, as indicated by the continuous line in the figure, — a result corre- 

 sponding exactly with what is observed to be in fact the case. Similarly in the 

 northern hemisphere, combining the proper motion of the cyclone, which on this 

 theory would be about north, -with the S.W. motion derived from the trade-wind, 

 we should have the actiial motion of the cyclone in a north-west direction, which 

 again corresponds exactly with the fact as obser\-ed within the tropics. Since the 

 causes above inferred to produce the proper motion of the cyclone would act less 

 powerfidly near the equator, it follows, on this theory, that the actual motion of 

 the cyclone would be slower there than at subsequent portions of its course, which 

 is constantly observed to be the ease ; and a chiirt of a remarkable stoiin, which 

 occmTcd near Mauritius in January 1855, was laid before the Section by the author 

 (see vol. iii. p. 23, ' Transactions of the Meteorological Society of Mauritius '), in 

 which this was manifested in a striking degree, in consequence, as supposed by Mr, 

 Ashe, of the influence of the ti-ade-wind being at a minimum at that season — a view 

 supported by the direction in which the storm travelled, which was S.S.E., corre- 

 sponding very nearly with the proper motion of cyclones in the southern hemi- 

 sphere, as deduced above. 



In all cases in which the influence of the ti-ade-wind is nil, whether owing to the 

 season or the latitude, we would expect that a cyclone proceeding rapidly polewards 

 would derive considerable easting from the circumstance of its having left lower 

 latitudes where the diumfil rotation is more rapid than in the places it is arriving at. 



Mr. Ashe regarded the recurving so constantly observed in cyclones in the South 

 Indian Ocean as being due to this circumstance, and exhibited a chart containing 

 the obser^-ed tracks of several cyclones (see chart i. vol. iii. of ' Transactions of the 

 Meteorological Society of Mauritius '), to show on the one hand that the recurving 

 is not O'wing to the presence of the island of Mauritius, as commonly supposed, 

 since several of the cyclones recurved in open sea far from land, and, on the other 

 hand, that the recurv ing was to be connected with the laritude, — as it occurred, as 

 shown in the chart, just where the component of motion due to the trade-wind was 

 vanishing, and the cyclone was assuming its own proper motion in a southerly 

 direction, with easting derived from the cause above mentioned. 



In the case of cyclones in the North Atlantic, the author referred the recurving 

 observed about the peninsida of Florida to the same causes, it being in about the 



