282 WRITINGS OF JOSEPH HENRY. [1855- 



with the polar current, descends to the surface of the earth, 

 (particularly along the continent of North America,) and 

 probably gives rise to the phenomena known by the name 

 of "Northers" and possibly also to the more violent north- 

 east storms of the coast. While the reversal of this system 

 takes place in one part of the earth, the more habitual 

 motion may be continued in another, and in this way a 

 mild winter in America, produced by a prevalence of south- 

 westerly wind, may be accompanied with a severe winter, 

 produced by northwesterly winds in some part of Asia, or 

 Eastern Europe. 



The belts and systems we have described are not station- 

 ary, but move north and south in different periods of the 

 year with the varying declination of the sun. For exam- 

 ple, the belt of rains is constantly almost directly under 

 the sun, and moves north and south with the changing 

 declination of that luminary, and thus divides the year in 

 the tropical regions into two rainy and two dry seasons. The 

 rain is produced (as has been abundantly shown) by the con- 

 densation of the vapor carried up by the ascending current of 

 air; the dryness on each side of this belt is the result of the 

 descent of the air which has been thrown out above, princi- 

 pally deprived of its vapor and increased in temperature both 

 by the heat due to condensation and to that absorbed before 

 it is thrown outward from the precipitated vapor. In the 

 summer season, when the sun is on the northern side of the 

 equator, the trade-wind system extends up on the ocean 

 sometimes as high as 40° N. latitude. A similar movement 

 takes place, but to a less extent, in the S3'stem of the tem- 

 perate zone. From this movement it is evident that there 

 is not only a variation of heat, but also of moisture and 

 precipitation at different seasons of the year. 



It is also necessary to mention that the belt of high barom- 

 eter is interrupted across the continent of North America, 

 and probably never passes farther north than the portion of 

 the United States bordering on the Gulf. But on this point 

 we cannot speak positively without more data and further 

 investigation. It is certain however that on the Pacific side, 



