2l6 



DEPTHS OF THE OCEAN 



top there are two glass bulbs, united by a bent capillary tube ; 

 the left-hand bulb is filled with creosote, the capillary tube 

 contains some mercury, and the right-hand bulb constitutes a 

 vacuum except for a little creosote. When the thermometer is 

 heated, the creosote on the left side expands, driving the 

 mercury through the tube so that it rises in the right-hand 

 branch ; the mercury lifts a small index, a pin 

 that is so constructed that it sticks at the place 

 where the mercury leaves it. When the ther- 

 mometer is cooled the creosote contracts, and the 

 creosote-vapours in the right-hand bulb propel 

 the mercury farther into the left-hand branch, 

 where there is a similar index. In this way the 

 index on the right shows the maximum tempera- 

 ture, and that on the left the minimum tem- 

 perature. The thermometer is fastened to a 

 rectangular plate carrying the temperature scale, 

 and the whole instrument is put inside a protect- 

 ing tube of copper. The maximum and minimum 

 thermometer needs about twenty minutes for 

 adjustment, and is slow enough not to change 

 appreciably during a rapid hauling up from 

 moderate depths, but if it has to be brought from 

 great depths, erroneous results may be recorded, 

 e.g. in waters where the temperature does not fall 

 or rise uniformly towards the bottom. In Arctic 

 and Antarctic seas, for instance, the temperature 

 generally falls to a minimum at about 50 or 70 

 metres below the surface, rising to a secondary 

 maximum at a depth of a few hundred metres, 

 finally falling again towards the bottom, and this 

 implies two maxima and two minima. In such a 

 case Six's thermometer would show only the 

 highest maximum and the lowest minimum en- 

 countered, and not the intervening values. This 

 thermometer has, however, done very good service ; it is, 

 for instance, astonishing how correct the temperature determina- 

 tions taken on board the " Challenger" have proved to be. In 

 the great depths of the ocean the variations of temperature from 

 year to year are so small that it is possible to verify now the 

 observations of earlier expeditions. 



The French physicist Aime about seventy years ago intro- 

 duced the reversing thermometer, which is caused — either by a 



HlI^SE 



Fig. 154. 

 Miller-Casella 

 Thermometer. 



