154 ANNUAL OF SCIENTIFIC DISCOVERT. 



succeed each other, the wind veers in general in the direction S., E., 

 N., W., S., round the compass. Exceptions to this rule are more com- 

 mon between N. and W. and between S. and .E., than between "W. 

 and S. or between E. and N. 



" This is the phenomenon which I have termed THE LAW OF GYRA- 

 TION." 



The trade-winds and monsoons are special causes of this law. Pro- 

 fessor Dove enters into a minute examination of the phenomena of the 

 first, dividing what he calls the permanent winds into the under and 

 upper trade-winds. Both, he is disposed to regard, as imperfectly de- 

 veloped monsoons, the word monsoon being as he shows, derived 

 from the Arabic mausim, or season. This is confirmed by the fact that 

 where monsoons exist, there are but two oscillatory movements of the 

 atmosphere annually ; monsoons being polar and equatorial currents, 

 alternately, according to the season of the year ; so that their directions 

 in the northern hemisphere are N. E. and S. W., and in the southern 

 S. E. and N. W. Violent as monsoons generally are, they are tame 

 in comparison to cyclones, which are truly terrible. Fortunately, the 

 area of these hurricanes is comparatively limited, being confined to 

 the West Indian seas ; their usual course being in a parabolic curve, 

 having some point near Bermuda for its focus originating in the 

 Gulf of Florida and running along the coast of the United States, 

 following generally the course of the Gulf-stream. Cyclones, as their 

 name imports, are strictly rotatory, and they never deviate from the fol- 

 lowing rules. Cyclones in the northern hemisphere possess a motion 

 that is retrograde, or in the contrary direction to the hands of a watch, 

 whereas, those occurring in the southern hemisphere have a converse 

 motion. On the equator itself cyclones never occur. According to 

 Professor Dove, they are 



" Most common in the district between the S. E. trade-wind and 

 the N. W. monsoon, which is called the region of the ' variables.' 



44 The rotatory motion takes place in the direction from E. through 

 S. toward W. and N. The intensity of the cyclone increases regu- 

 larly toward its centre. At the centre itself there is a dead calm, and 

 the greatest violence of the storm is experienced at the edge of this 

 calm circle. The diameter of this circle is greatest when the storm is 

 just commencing. If the rotatory motion increases in violence, the di- 

 ameter of this circle is decreased to about ten or twelve English miles. 



^D 



" The advance of the cyclone, up to lat. 20 S., is at the rate of 200 

 to 220 miles in the twenty-four hours. From that point it becomes 

 less rapid up to the outer edge of the S. E. Trade. The direction of 

 the advance is from lat. 10 S.,near the Indian Archipelago, to lat. 28 

 or 30 on the east coast of Africa ; first toward W. S. W., then to- 

 ward S. W. by S., and lastly toward S. S. W. 



" Throughout the whole of the cyclone, torrents of rain fall, which 

 arc more violent in front of it than behind it. The clouds are dark, 

 massive, and lead-colored, as the centre is approaching. Electrical ex- 

 plosions are most frequent on that side of the cyclone which is nearest 

 to the equator. The sea is disturbed irregularly to the distance of 300 

 or 400 miles during every such storm. The barometer falls rapidly as 

 the centre of the cyclone approaches, but the lowest level appears to 

 occur a little before it passes." 



