MOVEMENTS BENEATH THE SURFACE 129 



the tail is also being flexed, and its rising out of the water, as noted in the figures, may be regarded as 

 a consequence. The resuhing posture probably resembles that shown in Fig. 5^, and the whale would 

 appear to have gained a position of mechanical advantage : almost as much is hinted by Racovitza 

 (1903, p. 36). In this position, the flukes are sufficiently buried to have purchase for the final dive, 

 while the flattened head and the broad shoulders reaching downwards afford a bearing against which 

 force can be transmitted. 



Fig. 5. Diagrams illustrating the posture of Fin whales in ten stages while breaking surface and submerging at medium speed. 

 The arrows in e-i indicate the supposed direction of the beat of the tail flukes. 



The head, dorsal fin and caudal peduncle 



The head in rorquals may thus function as a vane and may perhaps be compared to the outstretched 

 hands of a diver or to the elevators of a submarine. In the vertical plane its effects would thus appear 

 analogous with those shown by Gray (1933, pp. 115-24) to result from lateral turning of the head 

 in fish. 



Authors have usually found difficulty in ascribing an appropriate function to the Balaenopterid 

 dorsal fin, and Howell (1930, p. 59), while suggesting its possible use as an equilibrator, also points 

 out the difficulty of accepting this conclusion about an animal swimming by a vertical motion of the 



tail. 



The fact observed above that the fin is usually the last part of the body to sink beneath the surface 

 and that it makes no splash, suggests strongly that it is streamlined for the purpose and that it facili- 

 tates submergence. Closer examination shows that the leading edge of the fin is normally set at an 

 angle of about 150° to the back anterior to it, but the angle is greatly increased when the back is arched 

 so that the edge of the fin tends to come into line with the curve of the back. The broad lines of the 



