31G PROCEEDINGS OP THE AMERICAN ACADEMY. 



cold-air and the warm-air cyclones respectively. These causes must 

 produce entirely or in part the well-known double diurnal period in air 

 pressure. At any rate, in view of the fact that an eclipse causes a cy- 

 clone over half a hemisphere, it will be necessary before rejecting such 

 a theory to show that the fall of temperature at night does not produce a 

 cyclone, or that this cyclone and the corresponding warm-air cyclone of 

 the day do not appreciably influence the barometer. 



The points in favor of the theory that the double diurnal period in 

 pressure is due to two diurnal cyclones, one developed by the cold of 

 night and the other by the heat of day, may be stated in brief as follows. 

 The theory is based on well-known physical laws. The possibility of a 

 cold-air cyclone under conditions similar to the diurnal cyclone is con- 

 firmed by the eclipse cyclone. The theory explains the annual oscilla- 

 tion of the time of maxima and minima of pressure in the diurnal period; 

 and explains the occurrence of a third maximum in high northern lati- 

 tudes in winter. The theory also explains why the warm-air cyclone is 

 well developed over continents, and on clear days, and causes a marked 

 fall in the barometer during the afternoon, while the morning minimum 

 of pressure over continents does not attain an excessive development as 

 compared with that over oceans where there is slight retardation of 

 the air movements on which the fall of the barometer in the cold-air 

 cyclone depends. 



The diurnal cyclones move from east to west, contrary to the motion 

 of ordinary cyclones in temperate latitudes. Their velocity of motion is 

 about one thousand miles an hour at the equator, and diminishes toward 

 the poles. The two charts in Figure 6 indicate the circulation of the 

 surface winds and upper currents in the diurnal cyclones. In these 

 charts the ordinates represent the hours of the day, and the abscissas 

 represent distances from the equator. The data for the surface winds 

 are derived from observations at Blue Hill, lat. 42° 13' N., long, 71° 7' 

 W., and Cordoba, Argentina, lat. 31° 25' S., long. 64° 12' VV.* The 

 directions of the arrows represent in the usual way wind directions, and 

 the position of the arrow shows the time of maximum frequency of each 

 wind. Thus the greatest diurnal freciuency of southerly winds occurs at 

 Cordoba at 7 a.m., and at Blue Hill between 7 and 8 p.m. There is 

 also a second maximum frequency of southerly winds at Blue Hill about 

 10 a.'m. The wind arrows at Cordoba and Blue Hill are, in general, 



* Annals of the Astron. Observatory of Harvard College, XXX. I't. iv., 415 and 

 419. 



