THE THEORY OF SUBSIDENCE 213 



THE EVIDENCE FOR THE THEORY OF SUBSIDENCEi 



It will be convenient to summarize the evidence scattered among the earlier sections 

 and discussed above in support of and against a theory that an onset of contrary condi- 

 tions may be followed by some subsidence of the upwelled water. 



According to Coker (191 8), fishermen have long believed in a swinging out to sea of 

 the current to explain the disappearance of cool water from the coast ; and its occasional 

 disappearance is noted in sailing directions. In earlier pages (pp. 206-9), such dis- 

 appearance of the cool water has been supposed possible, not by a horizontal swing but 

 by a vertical swing within the current. 



At Antofagasta and the Guafiape Islands, the rise in temperature was rapid but so 

 also, generally speaking, were the horizontal currents which might therefore have been 

 the cause ; at Antofagasta by a cyclonic inflow of warm water from the north, and at the 

 Guafiape Islands by an anticyclonic outflow of cold water to the north-west. 



At Antofagasta, (i) the drift of the ship against the wind and towards the shore at 

 St. WS 630, and (2) the rise in temperature simultaneously with a falling off of the wind 

 strength well before the subsequent change in its direction (Sts. WS 629-630), are to- 

 gether facts strongly in support of the subsidence theory (see p. 142). For the rise in 

 temperature between Sts. WS 629 and 630 might alone have meant nothing more than 

 the admixture of upwelled with oceanic water. The magnitude of the shorewards drift 

 at St. WS 630 at 17-26 miles from land could not easily be attributed to the coastal eddy 

 by which the inrush of warm water off Bahia Herradura may be explained (Figs. 9 

 and 28). The fact that the second line of observations lay in the region of eddy and not 

 over the first line weakens the value of any evidence that might be furnished by com- 

 parison of subsurface isotherms on these two lines. At the present stage the evidence 

 may be considered insufficient to decide how much of the rise in temperature on the 

 second line was due to subsidence and how much due to admixture with oceanic water 

 and the eddy. It is certainly suggestive that the three mechanisms were in operation. 



At the Guaiiape Islands the continuance of northerly current after change of the wind 

 may have been caused by aspiration from the north : and the conditions bore some re- 

 semblance to those at Arica, where the seemingly paradoxical co-existence of con- 

 vergence and divergence has been noted. The evidence is clearly insufficient to decide 

 the question whether some subsidence had occurred as a result of the wind change. 



Three alternatives have been suggested as the possible causes of the temperature 

 oscillation at Callao: the action of local wind; a shift in the position of the anticyclonic 

 swirls, with especial reference to the point of convergence of the wedge ; and the action 

 of seiche. The first was considered insufficient, alone, to cause the temperature changes, 

 while of the other two the data are inconclusive. While any horizontal current off 

 Callao must have been very slow (p. 129), the temperature changes also were very slow 



1 The term "subsidence" here denotes a reversion of the water layers towards a condition of horizontal 

 stratification and should be distinguished from sinking brought about through accession of density. 



14-2 



