-1859] WRITINGS OF JOSEPH HENRY. 81 



influence of which is strongly marked by the prevailing 

 winds. In a like manner the direction of the wind on the 

 coast of the Pacific is modified by the trend of the coast and 

 the parallel mountain chains. Almost every position at 

 which meteorological observations are made is liable thus 

 to be affected by the local topography ; but the result of this 

 is eliminated in a great measure by computing the average 

 direction from a number of stations within a limited dis- 

 tance of each other. Yet, though in this way the opposite 

 local influences in particular districts may be made to bal- 

 ance each other, those of great mountain systems still remain. 

 These in turn however may be merged in a series of obser- 

 vations extending across continents, or entirely around the 

 world. In this way, by collecting all the reliable observa- 

 tions which have been made on the winds in the northern 

 hemisphere, so far as they were accessible to the Smithsonian 

 Institution, Prof. Coffin has established the fact, before men- 

 tioned, that the resultant motion of the surface atmosphere 

 between latitude 32° and 58° in North America is from the 

 west, the belt being twenty degrees wide, and the line of its 

 greatest intensity in the latitude of about 45°. This how- 

 ever must oscillate north and south at different seasons of 

 the year with the varying declination of the sun. South of 

 this belt, in Georgia, Louisiana, &c., the country is in- 

 fluenced at certain periods of the year by the northeast trade 

 winds, and north of the same belt by the polar winds, which 

 on account of the rotation of the earth, tend to take a direc- 

 tion toward the west. It must be recollected that the westerly 

 direction of this belt here spoken of is principally the result- 

 ant of southwesterly and northwesterly winds alternately pre- 

 dominating during the year. 



From what has been stated in regard to the general circu- 

 lation of the atmosphere it would appear that these winds 

 are due to the returning upper currents which flow over from 

 the heated region of the equator, producing a southwest, a 

 west, or a northwest wind, according to the distance to which 

 they extend northward before they commence to descend to 

 the earth. If the sun continued on the equator during the 

 6-2 



