CYCLONES 33 



plainly showing that a depression was being generated between 

 Jamaica and the Cayman Islands, where several other de- 

 pressions have been noticed to generate (or to develop) before. 

 On the afternoon of the 28th the depression suddenly started 

 on its course, at first north-east, and then north, according to 

 subsequent news. 



In these and other cases the records show no unusual 

 features immediately before the generation of the depressions. 

 The wind, the rain, and the fall of pressure all take place 

 together as a matter of course. 



(3) Fully developed cyclones appear in the West Indies 

 for the most part during the months of August, September, 

 and October only; they follow a west-north-west course at 

 first, then they turn north, and finally recurve east-north- 

 east, if their course is long enough to permit of these changes. 



If we look at the Pilot Charts published each month by 

 the United States Hydrographic Office, we shall see that when 

 the region of equatorial heavy rains between South America 

 and Africa reaches as far north as latitude 15 , cyclones 

 originate in about that latitude, but to the west of the region 

 of heavy rains, and then move ofl: on a westerly course. As 

 the diverting efiect of the earth's rotation upon currents of air 

 is very important for the development and maintenance of 

 cyclones, and as this effect varies as the sine of the latitude, 

 there are no cyclones near the equator, or within 12" of it ; 

 but, as we have seen at 15^, the effect is sufficient to give the 

 currents the necessary divergence. Now, as the region of 

 heavy rains advances as far north as latitude 15^ in August, 

 somewhat farther in September and October, but withdraws 

 far to the south in November, and remains there until the 

 following July, it is evident that August, September, and 

 October are the months in which cyclones usually occur in 

 the "West Indies. Of course, they may occur at other places 

 and at other times if all the essentials are present and 

 combine. 



With regard to the course taken by cyclones, no doubt they 

 follow the general atmospheric drift, and move round the 

 anticyclone in the North Atlantic ; a large number pass over 

 the Bahama Islands, and a few pass over the Caribbean Sea. 

 Both the hurricanes of October 3, 1780, and August 18, 



