38 



OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 



scale by comparison with a thermometer of known accuracy, and it 

 should be read with the scale immersed to the height of the mercury 



column. Observations of sur- 

 face temperatures made upon 

 ill Pi g I bucket samples should be ob- 



I tained immediately after the 



I |W 1/ ^\ sample is taken, because heat- 



ing or cooling of the water 

 sample by radiation, conduc- 

 tion, and evaporation may have 

 a measurable effect upon the 

 temperature. From a vessel, 

 samples must be taken as far 

 away as possible from any dis- 

 charge outlets from the hull, 

 and, if the vessel is under way, 

 they should be taken near the 

 bow so as to avoid the churned- 

 up water of the wake. 



Protected Reversing 

 Thermometers. Reversing 

 thermometers (fig. 4) are usu- 

 ally mounted upon the water 

 sampling bottles (fig. 5), but 

 they may be mounted in re- 

 versing frames and used 

 independently. Reversing 

 thermometers were first intro- 

 duced by Negretti and Zambra 

 (London) in 1874, and since 

 that time have been improved 

 so that at present well-made 

 instruments are accurate to 

 within 0.01°C. On the Chal- 

 lenger Expedition the sub- 

 surface temperatures were 

 measured by means of mini- 

 mum thermometers. 



A reversing thermometer 

 is essentially a double-ended 

 thermometer. It is lowered 

 to the required depth in the set position (fig. 4), and in this position 

 it consists of a large reservoir of mercury connected by means of a 

 fine capillary to a smaller bulb at the upper end. Just above the large 



Fig . 4 . Protected and unprotected reversing 

 thermometers in set position — that is, before 

 reversal. To the right is shown the con- 

 stricted part of the capillary in set and reversed 

 positions. 



