PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 933 
the example given on page 931 we read dynamic meters instead of units of thick- 
ness, the corresponding units of pressure will be 50 decibars. 
If the dynamic depth to which it is necessary to descend into the sea to reach 
a given pressure be greater at one station than at another (as is necessarily the case 
if the specific gravity of the water varies regionally), only two alternative states are 
possible: (1) If the surface of the water is level, the given isobaric surface (surface 
at which the pressure is equal) must slope; or (2), if this isobaric surface is level, 
the surface of the sea must slope. The resultant circulation will differ accordingly. 
If the first alternative actually prevailed, the obliquity of the isobaric surfaces 
would increase with depth and the dynamic circulation would be most rapid at 
the bottoms of the deepest oceans. However, as Sandstrom (1919) and Smith (1926) 
both have emphasized, this is directly contrary to the truth, for the bottom waters of 
the ocean show only very slight regional variations in specific gravity and move only 
with inconceivable slowness. Consequently, when a dynamic gradient exists over 
any part of the sea it is the surface that slopes. It is of the greatest importance to 
keep this concept constantly in mind, because the conventional dynamic representa- 
tions in profile show the surface as level, and hence are likely to prove misleading. 
If, then, the isobaric plane chosen as the base for reference in our calculations 
lies so deep that it is level, or virtually so, calculation of the thickness of the column 
of water necessary to effect this pressure for a number of stations shows the actual 
contour or shape of the surface of the sea. Dynamic-contour charts of the deep 
oceans, such as have been constructed by Helland-Hansen and Nansen (1926) and 
by Smith (1926), are cases in point. In shoaler waters, however, where surfaces of 
equal specific gravity, and consequently the isobaric surfaces, are oblique right down 
to the bottom, the calculated dynamic slope of the surface of the sea will either 
exaggerate or minimize the true slope of the latter. 
This is the case in the Gulf of Maine. Consequently, the dynamic charts offered 
here can be taken only as a rough approximation to the state actually prevailing. 
The actual charting of the dynamic gradients in horizontal projection is hardly 
as simple as the foregoing resume might suggest because of the necessity for inter- 
grating the individual values for specific gravity at the levels of observation to 
arrive at the mean values for the included intervals; because, also, the specific 
gravities must be converted into specific volumes, and because the latter must be 
corrected for compression. The last two steps, however, are robbed of all difficulty 
by Hesselberg and Sverdrup’s (1915) tables, as simplified by Smith (1926, p. 18, 
Tables 3 and 4). Smith (1926) has so fully explained the construction of the dynamic 
chart, as well as the principles involved, in a publication universally accessible, that 
only one aspect of the procedure needs further comment here, namely, the modifica- 
tions necessary in studying an area so shoal and with stations differing so widely in 
depth that it is not possible to refer all the calculations to any one isobaric base 
plane. In this case it is necessary to calculate the gradient between pairs of adjacent 
stations, afterwards referring all to some one chosen station. Furthermore, if the 
specific volumes of the water at the two members of each pair of stations are not 
the same at the greatest depth reached at the shoaler, it is obvious that the inter- 
vening mass of bottom water deeper than that level must be in dynamic circulation; 
8951—28 60 
