1892.] On the Gran I Currents of Atmospheric Circulation. 45 



that from its upper part currents float away to both sides, northward 

 and southward : — and that these currents continue in the upper regions 

 of the atmosphere each of them advancing towards, and in part to, the 

 high latitudes of its own hemisphere, until, by cooling, its substance 

 becomes less buoyant, and sinks down gradually in various latitudes 

 of that hemisphere, and forms itself into a return current towards 

 the equator, in the lower part of the atmosphere. 



That the air of this great cap of atmosphere, covering the middle 

 and higher latitudes, and including portions of the currents just 

 described, having come from the equatorial regions, which were moving 

 absolutely from west to east in the earth's diurnal rotation with a 

 velocity of about 1000 miles per hour, must, on coming into those new 

 regions much nearer to the earth's axis, have greater velocity from 

 west to east than the earth below it in those new regions has. That 

 in the central or polar part of this great revolving cap of air the 

 barometric pressure must be abated in consequence of the centrifugal 

 tendency due to the extra speed of this great whirling cap of atmo- 

 sphere. That the bottom layers of this great cap of atmosphere, 

 being by friction on the earth's surface retarded as to this extra 

 Velocity of rotation eastward, must have a diminished centrifugal 

 tendency as compared with the quicker revolving air above them, 

 and, consequently, tend to flow, and actually do flow, inwards, 

 towards the region of abated barometric pressure at the centre of the 

 revolving cap of air. 



That thus, over the middle, or middle and higher latitudes, there 

 are three currents : — 



(1.) A top main current towards the pole. 



(2.) A bottom subordinate current towards the pole. 



(3.) A middle main current in direction from the pole, and con- 

 stituting the joint return current for both the preceding currents. 



And that all these three have a prevailing motion from west to 

 east, in advance of the earth. 



That the great return current, flowing in direction from the pole 

 towards the equator, arrives at a certain part of its course at which 

 it ceases to revolve eastward in advance of the earth; and, for the 

 rest of its course to the foot of the equatorial rising belt, it *blows 

 along the surface of the earth as the trade wind of the hemisphere in 

 which it is situated. 



The description here given of the author's theory, it is to be 

 noticed, is only a brief sketch. The aerial motions which have been 

 described are illustrated by the accompanying diagram (Plate 1) . The 

 arrows on the surface of the hemisphere represent the winds at the 

 surface of land or sea, not the currents in the higher regions. The 

 northward and southward motions, and the up and down motions, in 

 the main currents of the atmosphere, are indicated for all heights in 



