172 



DISCOVERY REPORTS 



was partly the result of vertical mixing is suggested by the greater uniformity of the 

 upper 60 m. and by the fact that in August water even at 120 m. was warmer than at 

 corresponding depths in July. Perhaps as a consequence of this lesser stability, up- 

 welling happens from greater depths in August than in July. The fact that the warm- 

 water wedge has moved closer inshore and has lost its former sharp distinction is also of 

 interest. In Table VIII the July and August lines are compared in greater detail. 



Table VIII. Seasonal differences in hydrological conditions off Callao 



Observations of a seasonal character were also obtained at the conclusion of the 

 survey on returning southwards in August and September. The surface temperature off 

 both Chile and Peru was cooler in August and September than earlier in the year. This 

 cooling is attributable to the effect of winter, but while it holds without exception for 

 the majority of the coast, the effect was not found between latitudes 34 and 39° S. Here 

 the surface temperature in September showed a rise over that of May. The comparisons 

 are made between observations in the same locality on the northward and the southward 

 journeys and only between those which were separated in time by more than three 

 weeks : twenty-two such series of observations are available, the broad results of which 

 are given in Table IX. The rise in temperature between 34 and 39° S may be corre- 

 lated with the Chilean monsoon, which will be shown on pp. 226-7 to be attended by a 

 change in the direction of the surface current. This observation, then, enables us to fix 

 the southern boundary of the Peru Coastal Current. 



Table IX. Changes in surface temperature from autumn to winter between 8 and 47° S 



