I90 DISCOVERY REPORTS 



plausibly the adverse current as an excuse for failing to arrive in Lima (1728, pp. 242- 

 243). Such is the accumulated experience of navigators, but northerly current, like 

 other currents on this coast, was pre-eminently irregular in its occurrence, a fact which 

 will have been noted in earlier sections and which, if borne in mind, will assist inter- 

 pretation of the hydrological conditions. 



According to Ocean Passages of the World {Somerv'iWe, 1923) the speed of the current 

 is 0-30 miles a day oflt Chile and 10-25 miles a day off Peru ; and according to Derrotero 

 de la Costa del Peru (Stiglich, 1918), the current can be neglected except in two places, 

 along the south coast of Peru northwards of Mollendo, and between Eten and Punta 

 Aguja. Our experience was much the same, and in this connection, the absence of current 

 off Callao which lay between these two centres is important, and seems to be a not 

 infrequent condition. Dinklage, while finding no current inshore at Callao, recorded 

 westerly set offshore (Schott, 1891). 



Since winds offshore are stronger, and more closely approximate to the south-east 

 trade, the lessening of northerly current with distance from shore is probably accom- 

 panied by an increasing set towards the west. The present survey gave few oppor- 

 tunities of observing current other than parallel to the coast, but the universality of 

 upwelling off both Chile and Peru is evidence that westerly set of the surface layers was 

 in progress and was widespread. It was especially pronounced off northern Peru 

 (Table I), the region where it is known to be characteristic (Garcia 1870, and the South 

 American Pilot, 1927) ; and it was also recorded by us off Antofagasta in the presence of 

 easterly and southerly wind. It was not, however, ubiquitous, and off the Lobos Islands, 

 San Juan, and Antofagasta (return journey) easterly set was met. The circumstances in 

 which easterly set was found suggest that it was of localized rather than of general 

 occurrence, and that it is usually bound up with eddies (e.g. off Antofagasta see pp. 

 127 and 208, and off the Lobos Islands pp. 131, 164 and 220). 



Deep Current. At the time of this survey, the convergence of sub-Antarctic water 

 and subtropical water in the meridian of 71-72° W. lay in 24-26° S. Although records 

 of surface drift give little evidence of northerly current^ in the sub-Antarctic water, a 

 study of salinity shows that the sub-Antarctic water extends northwards beneath the 

 subtropical water for some 10° of latitude: this has been shown on pp. 161-2 to differ 

 markedly from conditions in the South Atlantic where sub-Antarctic water returns 

 southwards shortly after meeting the subtropical water. Until the flow of subsurface 

 layers has been determined, the implication of this northerly extension of sub-Antarctic 

 beneath subtropical water on the west coast can be put forward only tentatively. Con- 



^ In these and other data, the strength of current is obscured by the difficulty of differentiating between 

 the effect of current and the effect of wind on the drift of a ship. Observations of current based on 

 discrepancies in the observed and calculated position of ships are easily vitiated by the effects of wind, 

 and windage cannot be recorded accurately because it varies daily with every ship (Lartigue, 1827, p. 21). 

 Only a ship which had steamed across the usual direction of the current or had been able to use a current 

 meter would have realized, for instance, the immobility of the water and the full extent of the wind met by 

 us off Pichidanque Bay; and in view of the prevalent weakness of the current on the entire coast, it seems 

 probable that popular belief in its strength may be exaggerated. 



