366 Biology in America 



in the box. Thus the direction of the current with reference 

 to the magnetic meridian is recorded by the number of shot 

 in the different compartments of the needle box. 



The early occanographers attempted to determine tempera- 

 tures in the depths of the sea either by bringing up a sample 

 of water from any given level in an insulated container and 

 recording the temperature of the sample on the deck of the 

 ship; or by insulating the thermometer itself so that it ac- 

 quired the temperature of the water very slowly, leaving it 

 for a time (several hours) at the desired level and then 

 hauling it up very quickly, so that its temperature would 

 change but slightly, if at all, in the ascent. The former 

 method of taking temperatures was employed by Nansen on 

 his polar expeditions and is still used to some extent, but 

 the principal method at the present time is the use of some 

 type of deep sea thermometer, which shall record the tem- 

 perature at a given depth, and give the same reading when 

 drawn to the surface. One of the earlier types of instru- 

 ment employed, and the one used by the "Challenger" expedi- 

 tion, was a modification of the ordinary maximum and mini- 

 mum air thermometer. For moderate depths, or in cases 

 where the temperature changes uniformly from the surface 

 downward, this type of instrument gives fairly satisfactory 

 results ; but for great depths, where the temperature does not 

 change uniformly from top to bottom, the results are un- 

 reliable. 



The later types of instrument used are all constructed on 

 essentially the same principle. They consist of a thermometer 

 with a neck which is twisted and very narrow just above the 

 mercury bulb. This is enclosed in a jacket of very heavy 

 glass hermetically sealed for protection against the enormous 

 water pressure at gi'eat depths, the mercury bulb being sur- 

 rounded by a special mercury jacket so that heat may be 

 rapidly conducted between the former and the water. After 

 lowering the thermometer to any given depth, it is reversed, 

 or turned upside down by a special mechanism, when the 

 mercury column breaks at the constricted point and drops 

 to the opposite end of the tube, where the reading is given 

 on a standardized scale. Before reversal the mercury can 

 pass up or down the tube through the constricted neck de- 

 pendent on the raising or lowering of the temperature ; but 

 after reversal and consequent breaking of the mercury column, 

 no more mercury can pass through and the reading gives the 

 true temperature except for a correction which has to be 

 made. During the ascent of the thermometer, if the difference 

 in temperature between air and water be considerable there 

 will be a slight change in the volume of the broken off column 

 of mercury and the reading will not be strictly accurate. A 



