246 



DEPTHS OF THE OCEAN 



itself, and all the materials carried into deep water, are com- 

 pressed. Water is, however, only to a slight extent compressible, 

 so the effect of pressure is not so great as is popularly supposed. 

 Tait and Buchanan have shown conclusively that compressi- 

 bility decreases slightly but sensibly with increase of pressure. 

 V. W. Ekman has recently made a very careful investigation on 

 the compression of sea- water, and has published Tables for Sea- 

 Water under Pre sszcre. From his tables we may easily compute 

 the actual density with compression, when depth, salinity, and 

 temperature are known. 



Let us take, as an example, the conditions at Station 63, 

 near the Sargasso Sea, 22nd June 19 10, as shown in the 

 following table, giving for the depths specified: (i) the 

 temperature, (2) the salinity, (3) the density disregarding the 

 compression (calculated by means of Knudsen's Tables), and 

 (4) the actual density with compression (calculated from 

 Ekman's Tables) : — 



It is seen that the density is practically identical, for instance, 

 at 3000 metres and at 4000 metres when leaving compression 

 out of account, whereas a considerable difference was actually 

 produced by the compression. At 4000 metres the effect of 

 the pressure of 400 atmospheres was so great that the density 

 increased from 1.02787 to 1.0462 1, equal to an increase of 

 weight of if per cent. As a matter of fact the water at 4000 

 metres has become only if per cent heavier by reason of the 

 compression ; a fairly delicate weighing would have been 

 necessary to detect this increase. The case may also be stated 

 thus : I litre of water at 4000 metres weighs 1046 grams ; if 



