340 DR ALEXANDER BUCHAN ON 



Thus, the station south of the equator indicates a very high salinity, which steadily in- 

 creases from the surface down to at least 100 fathoms, the station being within the region of 

 great salinity lying in the Pacific to the east of Australia. On the other hand, the station 

 north of the equator shows a salinity varying but little from the surface down to 400 

 fathoms. The only possible explanation is that at this station there is upwelling from 

 lower depths, bringing up to the surface water of comparatively low temperature and low 

 salinity so characterisic of the deeper waters of the ocean ; and in this connection it 

 falls to be noted that it is immediately to the east of this cold region where the east 

 flowing current of this part of the equatorial region of the Pacific has its origin. 



Analogous to this is the most striking illustration afforded by the ocean anywhere in 

 what takes place, in all probability, in intertropical seas from the bottom upwards. 

 This is in the lifting up, or upwelling, of the cold water beneath, by which is made good 

 the loss which takes place from the excess of the water lost from evaporation as compared 

 with what is received from the rainfall, and the loss occasioned from the powerful pole- 

 ward ocean currents of higher latitudes, which are originated and maintained by the 

 strong south-westerly winds of the Northern Hemisphere and the still stronger west- 

 north-westerly winds of the Southern Hemisphere. 



Conclusions. 



The prevailing winds, in their direct and indirect effects, are the most powerful agents 

 concerned in oceanic circulation. They originate and maintain the surface currents of 

 the ocean, and the influence of these currents is, through friction, felt to a depth of 

 probably several hundred fathoms. In intertropical regions the prevailing trade winds 

 drive the surface currents westwards to the eastern shores of the continents, and there, 

 accordingly, a greater depth of warm water is found occupying the upper layers of the 

 ocean than elsewhere ; and, except where the rainfall is abnormally heavy, this water is 

 not only very warm, but it has acquired from evaporation a salinity much higher than 

 the general average of the ocean. The results over the face of the ocean have been 

 already described, and may be studied in greater detail on Map 1 of the Challenger 

 Pteport. It is one of the most remarkable results of this inquiry that these areas of 

 high surface temperature and high salinity are found represented at all depths down to 

 the bottom, with just a tendency to an extension of the areas with increase of depth. 

 It follows that, waiving in the meantime what takes place at the bottom, the great mass 

 of the ocean intermediate between the upper layers and the bottom principally exhibits 

 vertical movements. 



( In the other hand, on the eastern sides of the oceans whence the trade winds start on 

 their course, there is an upwelling of the colder water of the greater depths towards the 

 surface in a manner similar to what Dr Murray has shown happens in the case of our 

 Scottish lochs when strong winds sweep over their surfaces.* In such cases the warmer 



* Proceedings, vol. xviii. pp. 142-43. 



