DISCUSSION AND ANALYSIS OF WINDS. 671 



is not from the tropical part of the ocean, it cannot bring much rain and pro- 

 duce the secondary areas of low pressure caused by condensation. Besides, the 

 region of high pressure on the Atlantic is far from the low pressure of Central 

 Asia, and near to that about Iceland ; so that the movement in the first direction 

 cannot be very constant. As to the air from over the lower latitudes of the 

 Atlantic Ocean and the Western Mediterranean, it is attracted towards Africa, 

 which is highly heated in summer, and open to the winds from the surrounding 

 seas. 



The geographical features of the North American continent explain why pres- 

 sure and winds are so different over it from what is seen in Asia. 



The coldest region of America is known to be to the north of the continent, on 

 the islands and ice-bound seas and sounds north of 70°. Ice and snow being bad 

 conductors of heat, the streams of warmer water are thus eff'ectually prevented 

 from having an influence on the air, and the ice-bound seas to the north of 

 America can cool as well as continents. 



But, as the coldest space north of the American continent is not separated by 

 mountains and plateaus from the surrounding regions, there cannot be such a 

 constant high pressure there as on the corresponding coldest space of Asia. It 

 will be remembered theft the lowest pressure of the northern hemisphere, especially 

 in winter, exists near Iceland, which is partly due to the warm waters of the Gulf 

 Stream. The coldest regions of America are not separated by any natural barrier 

 from this space, and thus air, even from the lowest, heaviest strata, should flow 

 towards Iceland. That this is the case, is shown by the winds in Greenland and 

 on the most northerly stations of the American continent ; they are northerly to a 

 very large extent. Probably the easy intercommunication between the coldest region 

 of North America and the region of low pressure near Iceland, explains why the 

 former has not a high mean pressure in winter. Having not a constantly high 

 pressure, the polar regions of America cannot influence the winds in the temperate 

 and tropical regions of this continent as the coldest region of Siberia, with its 

 constantly high pressure, does influence the temperate and tropical regions of Asia. 

 Next, we find a generally high pressure to the south of the United States, on the 

 Gulf of Mexico, as well as on the western highlands and plateaus of the conti- 

 nent, in lat. from 30° to 40° N. Probably, also, pressure is high to about 60° 

 lat. N. on the eastern slope of the Rocky Mountains, where, the winter being 

 cold, the Eocky Mountains in the west not permitting the air in the lower strata 

 to flow towards the Pacific, and the depression about Iceland being far away, there 

 exist all conditions for a high pressure. But barometrical observations from this 

 region are wanting. 



Thus, the Mississippi Valley and seaboard of the United States have in winter 

 regions of high pressure to the S. and W. of them; i. e. they are exposed to the 

 influence of winds from different directions, of which those that come from the S. 

 are warm and laden with vapor, and thus able to sustain the precipitations 

 necessary to the progress of storm-centres, while the air from the W. and N. W. 

 is cold and dry. 



A country generally level, subjected to such different influences, must have a 



