342 Dr George Buist on the 



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air heated near the equator by the overpowering influence of the 

 sun, is expanded and lightened : it ascends into upper space 

 leaving a partial vacuum at the surface to be supplied from 

 the regions adjoining. Two currents from the poles towards 

 the equator are thus established at the surface, while the 

 sublimated air, diffusing itself by its mobility, flows in the 

 upper regions of space from the equator towards the poles. 

 Two vast whirlpools are thus established, constantly carry- 

 ing away the heat from the torrid towards the icy regions, 

 and these becoming cold by contact with the ice, carry back 

 their gelid freight to refresh the torrid zone. Did the earth, 

 as was long believed, stand still while the sun circled round 

 it. we should have two sets of meridional currents blowing 

 at the surface of the earth directly from north and south 

 towards the equator, in the upper regions flowing back again 

 to the place whence they came. On the other hand were the 

 heating and cooling influences just referred to to cease, and the 

 earth to fail in impressing its own motion on the atmosphere, 

 we should have a furious hurricane rushing round the globe 

 at the rate of 1000 miles an hour — tornadoes of ten times 

 the speed of the most violent now known to us, sweeping 

 everything before them. A combination of the two influences, 

 modified by the friction of the earth, which tends to draw the 

 air after it, gives us the Trade-Winds — which sweep round 

 the equatorial region of the globe unceasingly at the speed 

 of from ten to twenty miles an hour : the aerial current, quit- 

 ting the polar regions with the comparatively tardy speed from 

 east to west imposed on it by the velocity due to the 70th 

 parallel, is left behind the globe, and deflected into an oblique 

 current as it advances southward, till, meeting the current 

 from the opposite pole near the equator, the two combine and 

 form the vast stream known as the Trades, — separated in two 

 where the air ascends by the belt of variable winds and rains. 

 Impressed with the motion of the air constantly sweeping its 

 surface in one direction, and obeying the same laws of motion 

 the great sea itself would be excited into currents similar to 

 those of the air were it not walled in by continents, and sub- 

 jected to other control. As it is, there are constant currents 

 flowing from the torrid towards the frigid zone, to supply 

 the vast mass of vapour there drained off; while other whirl- 



