4 THE ROYAL SOCIETY OF CANADA 



appears or may perhaps be considered as existing in a modified form 

 over Northern Asia. The changes in the distribution of pressure 

 over the North American continent are of the same type as over Asia 

 and Europe. 



In the Southern hemisphere where the land areas are smaller, 

 the primary system of circulation is less affected by a secondary 

 system, the sub-tropical belt of high pressure is more pronounced 

 and persistent, resulting in a more regular atmospheric circulation 

 than in the Northern Hemisphere. 



In 1903, at the meeting of the B.A.A.S. at Southport, Dr. H. H. 

 Hildebransson read a paper on the general circulation of the atmos- 

 phere touching more particularly on the knowledge derived from cloud 

 movement, and from the earlier balloon records obtained by M. 

 Teisserenc de Bort. The paper was short and stated with wonderful 

 clearness the conditions as we know them today after another ten 

 years of upper air exploration by kite and balloon. 



Dr. Hildebransson summarises his paper as follows: 



1. Above the thermal equator and the equatorial calms there exists 



an easterly current throughout the year. 



2. Above the trades there prevails an anti-trade from S.W. over the 



Northern Hemisphere and from the NW. over the Southern 

 Hemisphere. 



3. This Anti-trade does not pass the Northern limits of the trade 



wind. It is deviated more and more to the right over the Northern 

 Hemisphere and to the left over the Southern Hemisphere to 

 become a current from the west above the crest of the barometric 

 maximum of the tropics where it descends to augment the Trades. 



4. The entire zone of trade winds changes position with the seasons, 



moving northward and southward with the changing declination 

 of the sun, but through a smaller angle. 



5. From the tropical high pressure the atmospheric pressure diminishes 



continually towards the poles — at least beyond the polar circles. 

 Thus the air of the temperate zone is drawn in a vast polar whirl 

 turning from W to E. The turning movement seems to be of the 

 same nature as that of an ordinary cyclone; the air of the lower 

 strata approaches the centre and that of the upper layers departs 

 more and more from it with height above the earth's surface 

 up to the highest altitudes of which we have information. 



6. The strata of upper air of the temperate zones spread out above 



the high pressure of the tropics to descend there. 



7. The irregularities found at the earth's surface, particularly in the 



regions of the monsoons of Asia, disappear in general at the height 

 of the lower or intermediate clouds. 



