394 THE POPULAR SCIENCE MONTHLY. 



Without pausing here to examine this theory upon its merits and 

 upon the facts, we hasten to mention a different hypothesis advanced, 

 nearly two years ago, as a substitute for that of M. Dove, and as 

 affording an entirely original and satisfactory explanation of the 

 origin of cyclones. 



The hypothesis was likewise based upon the agency of the trade- 

 winds, but in a manner wholly different from that elaborated by the 

 German meteorologist. In the original paper in which my views were 

 published, the following statement was made: "It can be demon- 

 strated that the origin of cyclones is found in the tendency of the 

 southeast trade-winds to invade the territory of the southeast trades, 

 by sweeping over the equator into our hemisphere.'''' 



The hypothesis advanced, in lieu of another seemingly less satisfac- 

 tory, claimed to rest upon observations conducted in the very region 

 most notorious for the generation of cyclones. 



To test this, we need only to examine the Atlantic trade-winds. 



Theoretically, physicial geography has generally represented the 

 motions of the atmosphere somewhat as is represented in the accom- 

 panying diagram of the winds, as projected by Prof. William Ferrel, 

 of Cambridge. The elaborate pages of Prof. Coffin, in his invaluable 

 volume on the " Winds of the Northern Hemisphere," as deduced from 

 myriads of observations, show that the graphic illustration furnished 

 by the following diagram is approximately correct. 



The region of the trade-winds, it will be seen, more than covers the 

 torrid zone of the earth, and all seasons of the year overlaps both the 

 northern and southern tropics. While this is theoretically true, and 

 is usually put forth as fact, it must be accompanied with one or two 

 important qualifications and additions. 



Let us see what these are : The well-known oscillation or swinging 

 of the belts of winds to and fro on the meridians, which is kept up in 

 never-ceasing response to the apparent annual motion of the sun as 

 he crosses and recrosses the equator, must ever underlie the conception 

 we form of the trade-winds and be perpetually present to the mind's 

 eye. This oscillation has never yet received the popular attention it 

 needs. The sun traverses (apparently) an arc of 23^ on either 

 side of the line; and we might, a priori, suppose that the thermal or 

 meteorological equator, the thermal or meteorological Tropics of Can- 

 cer and Capricorn, and all those phenomena which lie between them 

 and beyond them, move over an arc of as many degrees as they trav- 

 erse. Such an inference, however, is not borne out by observation, 

 and we propose to confine ourselves strictly to what may be proved by 

 observation. It is clear that the trade-wind belt does traverse or 

 vibrate over a wider zone than any physicist has yet assigned to it, 

 which is not more than ten degrees of latitude north and south respec- 

 tively of the Tropic of Cancer and that of Capricorn. These winds, 

 when first experienced by Spanish sailors, gave, to that portion of the 



