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bulletin: museum of comparative zoology. 



the surface had begun to grow warmer in Massachusetts Bay by this 

 date (p. 391). At Boon Island, however, it was not until iVpril 5 

 that the first sign of spring warming was evidenced by the equaliza- 

 tion of temperature (39°) from surface to bottom. From this time 

 onward, near the Isles of Shoals, there was a steady rise of tempera- 

 ture, which made itself felt first and most strongly at the surface, and 

 later, to a much smaller degree, at the bottom (fig. 4). But the 

 surface warming was very irregular, and often interrupted, and even 

 temporarily reversed, by climatic conditions. During the winter 



fath 



Fig. 4. — Temperature sections off Boon Island, March 29 (Station 5 W. W. 

 Welsli) ; April 4 (Station 7 W. W. Welsh) ; and near the Isles of Shoals, 

 April 13 (Station 11 W. W. Welsh); April 16 (Station 13 W. W. Welsh); 

 April 26 (Station 19 W. W. Welsh); May 5 (Station 24 W. W. Welsh), 

 May 13 (Station 29 W. W. Welsh) and May 14 (Station 30 W. W. Welsh). 



when the column of water is of nearly uniform temperature from the 

 surface downward, offshore winds have little effect on surface tempera- 

 ture, because although the surface water moves offshore, yet the 

 waters which well up from below to take its place are of nearly the 

 same temperature as those which they displace. But as soon as the 

 surface is appreciably warmer than the underlying waters, any up- 

 welling, or vertical mixing, is at once evidenced by a decided drop in 

 surface temperature. Along the coast, upwelling is usually the result 

 of northwest winds ; but any gale causes more or less vertical mixing 

 of the upper few fathoms by wave action. How active these disturb- 



