Oceanic and Atmospheric Currents. 53 



isobars shows that they form at the same time areas of high 

 barometric pressure round which the atmospheric currents 

 revolve — with the hands of the watch in the northern hemi- 

 sphere, against the hands of the watch in the southern hemi- 

 sphere, while the equatorial belt and the circumpolar regions 

 form areas of low barometric pressure. If we now compare a 

 chart of isobars and a wind chart with a chart of oceanic surface- 

 currents, we find that there is a remarkable agreement between 

 them. These currents are observed to revolve together with 

 the winds round the areas of calms and of high barometric 

 pressure placed near the centre of each oceanic basin, about the 

 parallel of lat. 30°. The conclusion at which we arrive, that the 

 winds are the cause of the surface-currents, seems obvious, 

 although we must not forget that even in the absence of wind, 

 the thermal circulation of the ocean would resolve itself into a 

 system of surface and under-currents. It may be more in 

 accordance with facts to suppose that atmospheric and oceanic 

 currents mutually act and react upon each other. A current 

 of water will either raise or lower the temperature of a stratum 

 of air with which it remains in contact, and thus cause an 

 inflow of air from neighbouring colder regions, or an outflow of 

 air into adjoining warmer areas. A current of air will induce a 

 surface-current in the stratum of water with which it comes in 

 contact, or accelerate the velocity of a surface-current already 

 existing, or change its direction, or arrest its motion. That 

 the winds are a direct cause of oceanic surface-currents is a 

 fact too well established by actual observation to require further 

 proof. Perhaps the most striking example of their agency 

 will be found in the complete reversal of the currents in the 

 regions of the Monsoons. 



There exists a second and hardly less important cause which 

 tends to modify the general system of atmospheric and oceanic 

 circulation. The sun, in its apparent progress from tropic to 



