186 Mr Joseph John Murphy on the Circulatio7i 



vortexes from having so great an effect as this ; but their 

 centrifugal force does produce a sensible effect in keeping 

 the air away from their centres, and heaping it up at their 

 margins. The barometer stands at a maximum at about 

 lat. 28°, from which it falls towards each pole. This de- 

 pression is much greater in the southern hemisphere than 

 in the northern ; in the highest explored latitudes of the 

 south, the barometer stands at least an inch below its mean 

 level elsewhere. The reason I assign for this difference is, 

 that the vortex is much more perfectly formed in the 

 southern hemisphere than in the northern, owing to the 

 unequal heating of the continents and oceans in the latter, 

 which produces cross currents. Of course, the centrifugal 

 force is chiefly due to the velocity of the upper strata, as 

 that of the lower is reduced by friction against the earth's 

 surface. 



The excess of barometric pressure at lat. 28° over that at 

 the poles, and the comparative absence of centrifugal force 

 at the earth's surface, determine a motion of the air from lat. 

 30° towards each pole, and thus are produced the south- 

 west winds of the middle latitudes of the northern hemi- 

 sphere, and the north-west winds of the southern.* But 

 these can occupy only a comparatively thin stratum. In 

 the highest strata of every latitude, polar as well as equa- 

 torial, there must be a flow of air from the hotter to the 

 colder regions, from the equator to the poles, and a return 

 current underneath it, in the contrary direction. In the 

 circumpolar vortex, consequently, there is a motion from 

 the equator above and below, and a motion from the pole 



*■ The cause of the polar depression of the barometer was first, I believe, 

 pointed out by me in a paper read at the Belfast Natural History Society in 

 the winter of 1855-6, The whole theory of atmospheric circulation in extra- 

 tropical latitudes was first cleared up by Professor James Thomson, in a paper 

 on the " Grand Currents of Atmospheric Circulation," read at the British 

 Association in 1857, of which an abstract is published in the Transactions for 

 that year. His discussion of the subject is also published with some fuller 

 particulars in the " Proceedings of the Belfast Natural History Society " for 

 6ih April 1859. I never published my paper, as I afterwards became con- 

 vinced that it contained serious errors, but Professor James Thomson, in the 

 last-mentioned paper, has referred to me as having first explained the cause 

 of the polar depression of the barometer. 



