BIGELOW: OCEANOGRAPHY OF MASSACHUSETTS BAY. 401 



of the water. For the latter is diminished by surface cooHng as the 

 season advances, while it is, of course, progressively lessened as the 

 column of water becomes more and more uniform in temperature and 

 salinit}', until, bj' the end of November, it no longer opposes any 

 barrier to ^'ertical 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 jVIarch, 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 (sm-face 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, i. 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 A'ertical 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 tliroughout 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 



