PHYSICAL OCEANOGRAPHY OP THE GULF OF MAINE 
925 
The vertical stability varies little from season to season in the bottom stratum 
deeper than 100 meters, indicating comparative uniformity in the activity of vertical 
circulation there; but wide seasonal fluctuations in the stability of the superficial 
stratum reflect corresponding differences in the stirring effects of the tides, etc. 
Fig. 185.— Difference in density between the surface and the 40-meter level for May, 1915 and 1920. Corrected 
for compression 
In northern coastal waters generally (the Gulf of Maine is no exception) vertica 
mixing of the upper 100 meters is most active during the coldest season, when, thanks 
to low surface temperature, the water has little stability; and it is at this season 
8951—28 59 
