14 



surface, was a level plane. Other observations in this locality indicate 

 that the 750 isobaric plane, however, is not always level, but a 

 motionless state probably lies at some greater depth. The depth of 

 750 decibars, nevertheless, approaches most nearly to the level where 

 absence of motion may prevail of any depth of which the International 

 Ice Patrol records ; therefore, it has been employed in this paper as 

 an illustration of the most accurate base upon which to calculate 

 surface currents in the vicinity of the Grand Banks. 



7SO x)-e>.^f«-<£'(/-<E-TeK-&). 



Fig. 5.— The decrease in obliquity of observed isobaric surfaces with the observed increase in depth 

 and based upon the assumption that the depth of 750 decibars was a level plane in which no motion 

 prevailed. The figure includes a line of stations, 206 to 201, taken by the International Ice Patrol 

 south of Newfoundland May 5-7, 1922 



The position of a level surface depends solely on the acceleration of 

 gravity. Also, it has been pointed out that the depth to an isobaric 

 surface depends not only upon gravity, but upon the specij&c volume 

 of the overlying masses. Since we have already discussed gravity, let 

 us now turn to the remaining term, specific volume. 



SPECIFIC VOLUME 



Pressure per unit area depends upon two variables, gravity and 

 specific volume, but gravity being a more or less constant force, the 

 agency which exerts the greatest influence to vary the pressure 

 throughout the sea is specific volume. Specific volume has been 

 defined as the volume of unit mass of any body. It is simply the 



