The Lift Produced by the Heterocercal Tails of Selachii

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Abstract
1. The upward forces produced by tails of dogfish and tope, moving transversely through water at various speeds, have been measured. 2. The lift is approximately proportional to the 1·4 power of the transverse speed. 3. The equilibrium of dogfish and tope, swimming horizontally with the whole body horizontal, is discussed quantitatively. 4. It is estimated that in each case the hypocaudal lobe, acting passively, would give more lift than is required for equilibrium, at all swimming speeds greater than about 0·5 length/sec. 5. The lift may be adjusted to the required value by means of the radial muscles of the hypocaudal lobe.