364 



Forbes, Concerning the Flight of Gulls. 



rAuk 

 lJuly 



tance in front of the ship, (2) all along the windward side, (3) over 

 the smoke-stack, (4) astern, directly over the wake. 



Ascending currents of air must necessarily be produced by a 

 ship steaming to windward, and two distinct causes will be con- 

 cerned in their production. One is the diversion of the wind from 

 its natural course, both laterally and upwards, by the great mass 

 being forced through it. The other is the column of heated air and 

 smoke rising from the smoke-stack which must cause to rise with 

 it a considerable mass of the surrounding air. 



Most of the diversion of the wind by the mass of the ship is 

 probably lateral, but a fair proportion must be upwards. The 

 point where the upward diversion would be most felt would pre- 

 sumably be just above the windward side of the diverting mass. 

 But it would also extend a short distance to windward, for the ship 

 must drive before it a sort of cushion of air suffering compression 



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Fig. 2. 



from the approaching obstruction. Although the point of maxi- 

 mum upward diversion is presumably just over the weather bow, 

 there is probably an appreciable upward diversion from the propa- 

 gated obstruction some distance in front and to windward of the 

 ship (Fig. 2). 



Mr. Brewster doubted the existence of rising currents where the 

 birds were seen gliding fifteen or twenty feet above the upper deck 

 on which the wind seemed to blow horizontally upon him. I 

 think it is possible, and even' probable, that while the wind swept 

 the deck horizontally, it blew with an upward slant twenty feet 

 higher; for in meeting the vertical forward end of the upper deck 

 the air must have undergone some compression which reached a 



