BIGELOW: OCEANOGRAPHY OF MASSACHUSETTS BAY. 401 
of the water. For the latter is diminished by surface cooling as the 
season advances, while it is, of course, progressively lessened as the 
column of water becomes more and more uniform in temperature and 
salinity, until, by the end of November, it no longer opposes any 
barrier to vertical currents. 
After the water is practically uniform from top to bottom, a new 
cause for vertical currents is introduced, namely, the reversal of 
density consequent on further cooling at the surface. And after the 
beginning of December surface chilling was rapid, owing to the low 
temperature of the air, and to occasional falls of snow. As the winter 
progressed the surface temperature fell so fast that it was constantly 
coldest at that level, so that the water was in unstable equilibrium, 
aiding active vertical circulation which kept it thoroughly mixed un- 
til early March, when the first sign of spring was evidenced by a rise 
in surface temperature. The minimum temperature for the year was 
reached about the middle of February (surface 37.1°, 45 fathoms 
37.6°), and though data from north of Cape Ann is lacking for this 
month, it is probable that it was about equally cold over the whole 
region studied. By the middle of March the surface was once more 
as warm as the bottom in Massachusetts Bay, and vertical stability 
thus reestablished, for the surface was rather less salt than the 
deeper layers. But north of Cape Ann, 2. e., near Boon Island 
and Cape Porpoise, surface warming was not apparent until about 
two weeks later. Up to this time dynamic overturning, together with. 
the strong tidal currents, wave action, and the frequent upwelling. of 
bottom water near shore, must all be active factors in reducing the 
inequalities of temperature and salinity over the Gulf as a whole; 
while there is very little influx of shore water to hinder the process. 
But the sudden flooding of the surface with river water acts as an 
effective check to vertical circulation by lowering the surface density 
to such a degree that the water shortly assumes a state of pronounced 
vertical stability, constantly increased by the progressive warming 
at the surface, a condition which characterizes it throughout the 
summer. ‘The immediate result of this change is that surface warm- 
ing goes forward more and more rapidly, while the bottom tempera- 
ture, in 30-40 fathoms, rises so slowly that the difference between 
-- April and July at that level is only about 1°. And vertical movements 
are so much retarded that the surface freshening persists near the 
coast as late as August, although the river floods which cause it are at 
their maximum in April and May. 
The fact that the spring freshening was felt first near Cape Ann and 
