64 



BJ0RN HELLAND-HANSEN 



[REP. OF THE "MICHAEL SARS" NORTH 



ably considerable differences between the two stations 

 represented in Fig. 22. ^S has the following values: 



These values seem to indicate that water from the 

 Mediterranean made up a less prominent part of the 

 water-masses between 800 and 1200 metres at the "Mi- 

 chael Sars" station than at the other. 



For the stations represented in Fig. 23 we obtain 

 the following values of /\B: 



Here again, we have lower values at the station from 

 1910 than at the other station. The differences are however, 

 not so conspicuous as the differences in Ihe absolute 

 values of temperature and salinity. 



It is quite obvious that the physical conditions at 

 about 800—1200 metres in the region W. of the Bay of 

 Biscay are subject to great changes, either local or 

 temporal. The charts on pp. 96* and 97* and the more 

 detailed charts in Figs 28—29 are based upon all ob- 

 servations available regardless of possible annual or other 

 temporal variations (cf. section 36). No doubt, the local 

 variations at about 1000 metres may be very large in 

 many parts of the eastern North Atlantic. To a great 

 extent they are connected with the presence of water 

 from the Mediterranean and would be found by perfectly 

 synoptic observations too, even if some details in the 

 charts would then be altered. An advance or a retreat 

 of the water-masses in a horizontal direction, as well as 

 changes in their vertical position, may produce a great 

 effect at particular places (stations), and evidently such 

 changes may come under the category of annual variations. 

 Besides such fluctuations we probably also have variations 

 in the temperature of huge bodies of water in general, 

 and, as far as the eastern North Atlantic is concerned, in 

 the proportionate amount of water from the Mediterranean 

 that is present in the mixed water-masses at intermediate 

 depths. The consideration of these phenomena suggests 

 some interesting and important problems, but it would 

 lead us too far afield to enter into them here. 



We have chiefly discussed the conditions in the 

 eastern part of the North Atlantic, from which we have 

 a comparatively large number of observations. Even here, 

 however, a study of the annual variations must necessarily 

 be unsatisfactory; and this is still more true when we 

 turn to other parts of the North Atlantic, from which we 

 have no material at all for such studies. In the preliminary 

 report on the "Michael Sars" Expedition the present 

 author [1912] made a comparison between serial tempera- 

 tures taken by the "Challenger" and the "Michael Sars", 

 which seemed to indicate that the temperatures down to 

 about 1000 fathoms were lower, and sometimes very 

 much lower, in 1910 than in 1873 in the regions around 

 37^ N. and 48" W. This conclusion, however, is very 

 doubtful, as insufficient allowance was made for possible 

 local variations. 



34. Adiabatic Variations of Temperature. 



When a good insulating Pettersson-Nansen water- 

 bottle, which is fitted with a Nansen thermometer (cf. 

 section 11) and a reversing thermometer as well, has been 

 closed at a fairly great depth and hauled up quickly it 

 will be found that the two thermometers indicate diffe- 

 rent temperatures. The Nansen thermometer will show 

 a lower temperature than the reversing thermometer. An 

 observation, for example, from 4000 metres in the North 

 Atlantic will give a difference of about 0-45° C if the 

 insulation of the water-bottle is perfect. This is a result 

 of the diminution of pressure from about 400 atmos- 

 pheres to 1 atm., and is explained as follows: when a 

 compressible fluid has been exposed to pressure, and this 

 pressure is diminished, the fluid expands. The expansion 

 represents a quantity of work which is performed at the 

 cost of the heat energy in the fluid if no corresponding 

 quantity of heat is added from other, /. e. exterior, sources. 

 The temperature must consequently decrease when no inter- 

 change with the surroundings takes place. If, on the 

 other hand, a compressible fluid is exposed to increased 

 pressure, some energy is added to the fluid and mani- 

 fests itself by heating it. Such adiabatic variations of 

 temperature are well known in meteorology. Nansen 

 [1901, 1902] has introduced the problem of adiabatic pro- 

 cesses into oceanographic discussions. 



In our example with the insulating water-bottle not 

 only the water-sample but also the solid parts of Ihe 

 water-bottle are subject to adiabatic expansion. If it 

 were not for these solid parts, the decrease of tempera- 

 ture within the water-sample would be less than that 

 stated above, and amount to about 0-35° C. with a sample 

 lifted from 4000 metres to the surface in the North At- 

 lantic. This change represents the adiabatic variation of 



