-1859] WRITINGS OF JOSEPH HENRY. 53 



Every particle therefore of the current of air as it flows 

 towards the equator in the northern hemisphere would par- 

 take of the motion of the place at which it started, and in 

 its progress southward it would reach in succession latitudes 

 moving more rapidly than itself It would thus as it were 

 continually fall behind, and appear to describe on the sur- 

 face of the earth a slightly curvilinear course towards the 

 west. A similar result would be produced on the south side 

 of the equator; and hence we have the first conception of 

 the cause of the great systems of currents denominated the 

 "trade winds," blowing constantly within the parallel of 30° 

 from the northeast in the northern hemisphere, and from 

 the southeast in the southern, towards the belt of the greatest 

 rarefaction. 



The motion however will require further consideration. 

 The particles of air approaching the equator will not ascend 

 in a perpendicular direction, as was first supposed, but as 

 they rise will continually advance towards the west along an 

 ascending plane, and will continue for a time their westerly 

 motion in the northern hemisphere after they have com- 

 menced their return towards the north. They will how- 

 ever as they advance northward, arrive at parts of the 

 earth moving so much less rapidly than themselves, that 

 they will gradually curve around towards the east, and 

 finally descend to the earth, to become again a part of the 

 surface trade wind from the northeast. The particles will 

 tend to move westward as they ascend: first, on account 

 of their momentum in that direction; and secondly, be- 

 cause, as they reach a higher elevation, they will have less 

 easterly velocity than the earth beneath. They will also be 

 affected by another force, as has lately been- shown by Mr. 

 W. Ferrel, due to the increase of gravity which a particle of 

 matter experiences in travelling in a direction opposite to 

 that of the rotation of the earth. The last mentioned cause 

 of deflection will operate also in a contrary direction on the 

 atoms when they assume an easterly course. 



The result of the complex conditions under which the 

 motive power acts in such a case would be to produce a sys- 



