The Winds. 249 



nearly a quarter of an inch between the equator and the tropics 

 in this part of the ocean. 



A further account of the origin and nature of the great 

 ventilating system is given by Sir John Herschel in the same 

 work from which the former paragraph was borrowed (Meteor- 



ologV) P- 56) : — 



"The air of the cooler surrounding region not being so 

 relieved (by overflow), but rather the contrary, owing to the 

 increase of weight poured on it from above, will be driven in 

 by the difference of hydrostatic ■ pressure so arising ; and thus 

 originate two distinct winds, an upward one, setting outward 

 from the heated region, a lower, inward. If the region heated 

 be a whole zone of the globe, such as the generally heated 

 intertropical region, these currents will assume the character of 

 two sheets of air setting inwards on both sides below, uniting 

 and flowing vertically upwards along the medial line, and 

 again separating aloft, and taking on a reversed movement." 



The upper and lower currents, from what was said in the 

 former article on their stratiform character and horizontal 

 extension, remain distinct, and travel separately over a tract 

 of many degrees from the equator. The direction of the 

 upper current may be inferred from the following remarks of 

 Redfield, whose opinion on the subject of aerial currents must 

 always be regarded as of the greatest value : — 



" We learn from Humboldt that in the great eruption of 

 Jorullo, a volcano of southern Mexico which is 2100 feet 

 above the sea, the roofs of the houses in Queretano, more than 

 950 miles N., 37° E. from the volcano, were covered with the 

 volcanic dust. In January, 1835, an eruption took place in 

 the volcano of Cosequina, on the Pacific coast of Central 

 America, having an elevation of 3800 feet above the sea, the 

 ashes from which fell on the island of Jamaica, distant 730 

 miles N., 60° E. from the volcano. The ashes of the volcano of 

 St. Vincent fell at Barbadoes in 1812, a distance of 120 miles 

 nearly due east from the volcano." 



The direction of the upper current is therefore very oblique 

 to the meridian, and directed from south-west towards north- 

 east. The cause of this deviation now remains to be explained. 



Were the earth at rest, and did the sun actually revolve 

 about it as it appears to do, the main currents of the air 

 produced at the equator would everywhere follow the course 

 of the meridians, and no deviation would occur. But an 

 appeal to experiment will readily show that a deviation of 

 exactly the kind observed is really caused by the rotation of 

 the earth. 



Suppose a whitened globe to be suspended by one of its 

 poles ; in such a manner that it can be turned from east to west,. 



