PHYSICAL OCEANOGRAPHY 219 



ters in about 600 double determinations was only y-J^" C, so 

 that the temperature of the greatest ocean depths can now be 

 determined with great accuracy. 



A common form of reversing mechanism is a brass tube Reversing 

 which can turn over within a frame. A pin retains the tube "mechanism. 

 (into which the thermometer is placed) in an upright position ; 

 when the pin is withdrawn, the tube is tipped over by the aid 

 of a steel spring. The pin is removed either by means of a 

 propeller or by a messenger. The propeller is so adjusted that 

 it does not move during the descent, but when the apparatus 

 is pulled upwards it revolves, drawing out the pin along with 

 it. Formerly this propeller-release was employed with many 

 kinds of oceanographical apparatus, but it is not always reliable, 

 especially in a rough sea, and the apparatus must be hauled 

 up some distance before the propeller works. It is, therefore, 

 gradually being superseded by the messenger, a small weight 

 which is fixed on the line and let down after the apparatus has 

 reached the desired depth. When the messenger reaches the 

 reversing mechanism it knocks out the pin and the thermometer 

 is turned upside down. One of the wa£er-bottles used during 

 the "Michael Sars " Expedition is reversed together with the 

 thermometer ; in other words, this water-bottle is a reversing 

 mechanism for taking a temperature and a water-sample at the 

 same time. 



The Pettersson-Nansen water-bottle has a very high in- Pettersson- 

 sulating capacity, and the temperature of the water-sample is ^!au?.bottie. 

 not affected by conduction even when hauled up from a depth 

 of several hundred metres, though the apparatus may be 

 drawn through water-layers having very different temperatures. 

 Pettersson originally used an ordinary thermometer, which was 

 inserted into the water-bottle after it came up. Then Nansen 

 thought of fixing a thermometer inside the water-bottle, and 

 in this way the temperature at any depth was determined more 

 easily as well as more exactly. The Nansen thermometer is 

 very delicate, and is protected by a strong glass tube against 

 the great pressure. 



In making temperature-observations, however, one special Effect of great 

 precaution must be taken. When a liquid is exposed to great P'^'^'^^"'^^- 

 pressure its volume is slightly diminished, and, some heat being 

 liberated, the temperature of the liquid rises. Lord Kelvin 

 studied this question carefully, and arrived at a formula by 

 which such changes of temperature maybe calculated. Con- 

 versely, the volume of a liquid released from great pressure 



