250 



•450 



450 



observations from a sufficient number of levels of depth in order that 

 the change in physical character of the water may be followed in 

 detail. It is equally important that observations be extended down- 

 ward to abyssal regions where uniformity of conditions tend to pre- 

 vail. The greatest changes per unit increase in depth in an ocean 

 are generally in the surface layers, and the deeper we penetrate 

 the more homogeneous becomes the mass. A characteristic graph 



of the density which is 

 based on the two funda- 

 mentals, salinity and tem- 

 perature, is shown in Fig- 

 ure 27. The upper 25 

 meters is generally kept 

 more or less homogeneous 

 by the mixing effect of the 

 waves, a feature illus- 

 trated by the steepness of 

 the density curve. The 

 water column between 25 

 and 50 meters increases 

 in density very rapidly, 

 i.e., the water is very sta- 

 ble, and this is shown by 

 the horizontality of the 

 curve at A. We find a 

 secondary unevenness in 

 the curve between the 125 

 and 200 meters depth, B, 

 which is often observed 

 and attributed to the limit 

 of depth of the seasonal 

 effect. Below this point 

 the curve gradually and 

 constantly approaches a 

 straight line as homoge- 

 neous abyssal water is 

 entered. In accordance 

 with this normal stratifi- 

 cation, the ice patrol has adopted a minimum number of standard 

 depths at which the observations for salinity and temperature are 

 always taken, viz., 0, 25, 50, 125, 250, 450, and 750 meters. It has 

 been found, however, that considerable circulation takes place even 

 below 750 meters, if we proceed as far 120 miles offshore from the 

 continental edge. Therefore it would seem desirable in future years 

 to extend the observations at least to 1,200 meters. 



750 



Fig. 27.— An example of the distribution of density with ocean 

 depth 



