of the Atmospheres of the Earth and the Sun. 187 



at ail intermediate level. The motion from the poles is in 

 the direction of the centrifugal force, that from the equator 

 is against it. 



There are two regions of barometric maxima, lat. about 

 28° or 30°, and three of minima, at the equator and at the 

 poles. From the maxima, air f3ows at the surface of the 

 earth to the minima, appearing in the tropical regions as 

 the trade-winds, in the circumpolar as the counter-trades. 

 The polar minima are produced by centrifugal force, and a 

 barometer placed at any height above the sea-level, in the 

 region of the polar minimum, will consequently stand below 

 the normal level for that height ; for centrifugal force acts 

 at all depths in a vortex. But the equatorial minimum is 

 produced in a totally different way, namely, by the ascent 

 of rarefied air and outflow above. In that region, conse- 

 quently, a barometer placed in the lower strata, where the 

 currents are flowing inwards towards the barometric mini- 

 mum, or comparative vacuum at the equator, stands below 

 its normal level ; but if placed in the upper strata, where 

 there is an outflow of air towards the poles, it will stand 

 above its normal level for the height. This is because an 

 outflow can only be the effect of increased pressure and an 

 inflow of diminished pressure. 



In Plate I. the inner circle represents the earth, and the 

 circles concentric with it represent level surfaces in the 

 atmosphere. The dotted lines represent isobarometric sur- 

 faces. It will be seen that at all elevations they fall towards 

 the earth's surface in nearing the poles. At the lower 

 heights, they fall towards the earth's surface in nearing the 

 equator, but at the greater heights they rise higher in 

 nearing the equator. 



"Within the triangular spaces enclosed by the lines drawn 

 from west and east to the earth's surface, the wind is from 

 the east ; outside of these, it is from the west. 



The arrows marked a, indicate the trade-winds ; h, the 

 upper return trade-winds, which extend to the poles ; c, the 

 winds at middle height in the higher latitudes, which blow 

 towards the equator ; d, the winds at the earth's surface in 

 the higher latitudes, which blow towards the poles. 



It is obvious that in any other planet which rotates on 



