ane. 
13 
DEPTH AT WHICH GREATEST OBLIQUITY OF ISOBARIC SURFACES 
OCCUR 
It is important to distinguish where the greatest obliquity of the 
isobaric surfaces prevail in an ocean mass. Dynamic measurements 
and pressures have been considered as being laid off from the surface 
of the sea downwards on the assumption that the sea surface is always 
level—an equipotential surface. This premise demands considerable 
revision, as we shall see, in the light of the following facts: 
As aresult of compiled oceanographic observations, it is well known 
to-day that the greatest variations in temperature and salinity of the 
water take place in the upper levels of thesea. In the North Atlantic, 
for example, below depths of 3,000 meters there is little variation, as 
we proceed from place to place, in the temperature or the salinity. 
Now, if we regard two stations with widely differing specific volumes, 
we shall generally find that their difference decreases more or less 
rapidly with an increase in depth, and gradually approaches a constant 
or zero. Where the water is light we shall observe a relatively low 
pressure in decibars at a certain dynamic depth, or conversely at a 
given observed pressure in decibars, the dynamic depth will be least 
where the water is heaviest. In view of this natural state of the 
ocean, if the sea surface be level, then the obliquity of the isobaric 
surfaces must increase downwards and the maximum of forces and 
currents would be relegated to the greater depths, a condition which 
we know is contrary to fact. It follows alternatively that at an 
appreciable depth below the surface there will generally be a sheet 
where motion most nearly approaches zero and where isobaric, 
isosteric, and equipotential surfaces are parallel. It follows, further- 
more, that above such a motionless plane, the water, over any 
given horizontal extent, lies at the greatest height (the surface of 
the sea highest) at that place where the water is the lightest—i. e., 
the specific volume the greatest. 
We should endeavor to select from a group of observations indi- 
cative of a surveyed area an isobaric surface which in itself has the 
most nearly equal dynamic depths, thereby sounding out a level or 
motionless plane and which as stated before generally will be found 
to lie at arelatively great depth beneath the surface of the sea. When 
employed as a “bench mark” this surface provides a means of measur- 
ing the currents which usually are present in the upper levels. The 
velocities are determined by a comparison of any two dynamic heights 
measured upwards from the level, isobaric plane to the surface of the 
sea. Figure 5, page 14, shows in exaggerated form the obliquity of 
the sea surface, and also the other isobaric surfaces of observation as 
they lay May 5-7, 1922, south of the Grand Banks, between stations 
206 and 201. The state of relative obliquity is based upon the 
assumption that the maximum depth of observation, the 750 dec.bar 
