ASTRONOMY AND METEOROLOGY. 409 



clc, namely, that the prevailing winds on the surface of the earth in latitudes 

 higher than thirty degrees, arc, while blowing from the west, as should be 

 expected, found to blow more towards the poles than from the poles ; and 

 thus do not move as if impelled along the surface of the earth from polar to 

 equatorial regions by an augmented pressure due to condensation by cold in 

 polar regions, and a diminished pressure due to rarefaction in the equatorial 

 regions. Observations being thus at variance with the only obvious theory 

 proposed, the circumstance in question has been commonly regarded as 

 rather paradoxical ; and Lieut. Maury found himself forced into supposing 

 an entire reversal in latitudes above thirty degrees of the great circulation, 

 just described. Mr. Thomson regards Lieut. Maury's supposition as being 

 entirely unsupported by the known physical causes of the atmospheric mo- 

 tions. He, on the contrary, maintains that the great circulation already 

 described docs actually occur, but occurs subject to this modification, that a 

 thin stratum of air on the surface of the earth in the latitudes higher than 

 thirty degrees a stratum in which the inhabitants of those latitudes have 

 their existence, and of which the movements constitute the observed winds 

 of those latitudes being, by friction and impulses on the surface of the 

 earth, retarded with reference to the rapid whirl or vortex motion from west 

 to east of the great mass of air above it, tends to flow towards the pole, and 

 actually does so flow, to supply the partial void in the central parts of that 

 vortex, due to the centrifugal force of its revolution. Thus it appears that, in 

 temperate latitudes, there are three currents at different heights ; that the 

 uppermost moves towards the pole, and is part of a grand primary circula- 

 tion between equatorial and polar regions ; that the lowermost moves also 

 towards the pole, but is only a thin stratum forming a part of a secondary 

 circulation ; that the middle current moves from the pole, and constitutes 

 the return current for both the preceding ; and that all these three currents 

 have a prevailing motion from west to east in advance of the earth. 

 This is the substance of Mr. Thomson's theory ; and he gives, as an illus- 

 tration, the following simple experiment : If a shallow circular vessel, 

 with flat bottom, be filled to a moderate depth with water, and if a few small 

 objects, very little heavier than water, and suitable for indicating to the eye 

 the motions of the water in the bottom, be put in, and if the water be set to 

 revolve by being stirred round, then, on the process of stirring being ter- 

 minated, and the water being left to itself, the small particles in the bottom 

 will be seen to collect in the centre. They are evidently carried there by a 

 current determined towards the centre along the bottom, in consequence of 

 the centrifugal force of the lowest stratum of the water being diminished in 

 reference to a strata above through a diminution of velocity of rotation in 



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the lowest stratum by friction on the bottom. The particles being heavier 

 than the water, must, in respect of their density, have more centrifugal force 

 than the water immediately in contact with them ; and must, therefore, in 

 this respect have a tendency to fly outwards from the centre, but the flow of 

 water towards the centre overcomes this tendency and carries them inwards ; 

 and thus is the flow of water towards the centre in the stratum in contact 

 with the bottom palpably manifested. 



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