SALINITY 



103 



may be satisfactory, there is nevertheless a change of tempera- 

 ture, due to the diminution of pressure on the enclosed watfr. 

 This change of temperature is inappreciable in shallow water. 

 But in greater depths the release of pressure which takes place 

 during the hauling in lowers the temperature of the enclosed 



FIG. 16. THE PETTERSSOX-NAXSEX WATER-BOTTLE. 



sample. For that reason the thermometer inside the water- 

 bottle is not relied upon absolutely, but is compared with the 

 outside reading given by the inverted Richter thermometer. 



Surface Salinities. 



The Atlantic Ocean has the highest surface salinities. 

 These attain in the North Atlantic a maximum of over 37 per 

 mille from the Canary Islands to 55 W. Long., and on both 

 sides of the Tropic of Cancer from 17 to 30 N. Lat. the 



