-1859] WRITINGS OF JOSEPH HENRY. 55 



made out by Professor James H. Coffin in calculating the re- 

 sultant of all the winds of the northern hemisphere, after hav- 

 ing eliminated the eflects of extraneous action, and thereby 

 exhibiting the residue as the result produced by the gen- 

 eral circulation. 



Another condition however must be introduced. These 

 belts would not be stationary, but would move laterally to- 

 wards the south or the north, according to the varying posi- 

 tions of the sun at different seasons of the year. Their 

 breadth would also vary ; because they would be crowded 

 into a smaller space towards the pole in the winter, and ex- 

 panded into a wider space in the summer. 



To trace with precision the path which would be described 

 by a particle of air in its circuit, while under these vary- 

 ing perturbing influences, transcends the power of un- 

 aided logic, and could only be accomplished (if at all) by 

 means of the most refined mathematical artifices. This 

 problem has lately been presented (it is believed) as one of 

 the prize questions of the French Academy of Sciences. 

 Were it however solved with all the conditions that have 

 been assigned, this would not be sufficient ; since there is 

 another cause of disturbance, perhaps more active than any 

 yet enumerated, namely, the condensation of the vapor 

 which arises from the surface of the ocean and is carried to 

 different parts of the earth by the currents described. We 

 owe to Mr. Espy, of this country, the principal develop- 

 ment of the action of this agent in modifying and controll- 

 ing atmospheric phenomena. The heated air whicli ascends 

 at the equator is saturated with moisture, which it has ab- 

 sorbed in its passage over the northern and southern oceans. 

 As it ascends above the surface of the earth it meets contin- 

 ually with a diminished temperature; and as the sun daily 

 declines into the west, a considerable portion of it is con- 

 verted into water which returns to the surface in the form 

 of rain. The greatest effect of this action is immediateh'' be- 

 neath the sun ; and hence the belt of inter-tropical rains 

 oscillates to the north and south with the course of the sun 

 in its annual changes of declination. A portion however 



