•82 WRITINGS OF JOSEPH HENRY. [1855- 



year, and there were no obstacles to the free motion of these 

 currents, they would be constant in intensity and direction 

 around the whole earth ; but the change in declination of 

 the sun, and the obstacles opposed by continents and moun- 

 tain chains, modify in an important degree the simplicity 

 of this motion. When the sun ascends to the north, it 

 carries with it the whole circulating system of the atmos- 

 phere, causes the northeast trade winds to invade the south- 

 ern part of the United States, and the inferior currents, which 

 give rise to the southwest wind, to flow in summer over a 

 large portion of our territory. The latter, charged with 

 the vapor from the Atlantic and the Gulf of Mexico, impart 

 warmth and fertility to all parts of the surface on which 

 they descend. The higher currents, which produce the west 

 and northwest winds, flow in summer above us, to descend 

 further to the north. Their course however is marked by the 

 almost invariable direction of the upper clouds and of the 

 summer thunder storms, which, in the greater part of the 

 United States, pass from the west to the east. The curving 

 course of the returned currents, when the sun is south of the 

 equator, is perhaps best marked by the direction of the hur- 

 ricanes, which exactly follow the path we have described as 

 that of the particles of air in the general circulation so often 

 referred to. This will be seen by examining the storm tracks 

 on one of the maps of the lamented Redfield. 



It is evident, from theory as well as from every day obser- 

 vation, that the currents of the belt of the northern hemi- 

 sphere, in which the United States is situated, must be subject 

 to many perturbing influences, and that this region is well 

 entitled to the denomination of the zone of variable winds. 

 While the great circulation which we have described is 

 going on, particularly above us, every rain that occurs and 

 every variation of temperature tends to disturb its regularity 

 at the surface of the earth. According to the views here 

 presented the following winds of the United States belong to 

 the general circulation, namely, the southwest, west, north- 

 west, north, and northeast; while those from the opposite 

 quarters of the horizon are principally due to abnormal 



