470 Prof. Dove on the Changes of Wind in a Cyclone, 



the two opposite sides of the middle line. In a cyclone advan- 

 cing towards the north (in the northern hemisphere), the change 

 of wind on its eastern side always takes place from E. through 

 S. to W., or " with the sun -/' on the western side of the storm, 

 from E. through N. to W., or "against the sun/' Now if all 

 changes of wind were more or less direct results of advancing 

 rotatory storms or cyclones, there would necessarily be found in 

 the northern hemisphere as many cases of change of wind of one 

 of these ways as in the other ; for although the cyclones prevail 

 and pass over certain parts of the ocean more especially, yet the 

 preponderance of one direction of the change of wind in one 

 place would be compensated by the opposite preponderance in 

 another, and on the whole there would not remain any prepon- 

 derance. 



I have, however, shown by special investigations, that there 

 is such a preponderance, and that it is the case in both hemi- 

 spheres; that in the non-tropical zone of the northern hemi- 

 sphere the wind in the majority of cases changes from S. through 

 W. to N. and E., and in the southern hemisphere from S. through 

 E. to N. and W. ; this demonstrated preponderance must there- 

 fore have some other cause, and that cause must be one of 

 general conditions, because it produces opposite effects in oppo- 

 site hemispheres. The space between two meridians is an 

 equilateral triangle, having its base on the Equator and its sum- 

 mit at the Pole. It is evident that the whole mass of air which 

 ascends over the base line in the meridian of calms cannot flow 

 in the rapidly narrowing space up to the point at the summit of 

 the triangle, but that it must ascend again before reaching that 

 point. 



This descent takes place at the outer limit of the "trade 

 winds,'' but at variable points ; so that while at certain places 

 portions of air are flowing from the upper regions of the tropical 

 atmosphere into the temperate zone, at other places at the same 

 time the contrary may be taking place, causing an extension 

 backwards of the " trade wind," and making it appear to begin 

 earlier than would correspond to its mean outer limit. 



The currents of air which in the tropics flow over each other, 

 in the temperate zones flow alongside each other and in variable 

 beds. Our phsenomena of weather, therefore, at each particular 

 place of observation depend on two currents, which alternate as 

 each in turn prevails over and presses aside the other, or drives 

 it off the field. When one of these currents prevails in full in- 

 tensity, we have the extremes of climate ; where they strive for 

 the mastery, we have its variability. Let us now suppose that 

 in London a northerly current of air begins, that is to say, that 

 the mass of air between London and a point to the north of the 

 8hetlands is set in motion ; an observer in London will remark 



