82 
A VOYAGE TO SPITZBEBGEN. 
rates of speed. Swimming gently along, the fan-like 
tail moves with a regular rhythmical motion, giving 
sufficient force to drive the weighty body in the desired 
direction; but when roused to action this powerful 
organ is driven with enormous muscular force; then the 
lobes are more rigidly exerted, and the body acquires 
observe the action of the enormous tail, which differs 
from that of fishes, in being set on horizontally 
instead of perpendicularly. There is a slight curve 
in the surface of each flange, imitated, no doubt, in 
the construction of a screw propeller for a steamship. 
By working this enormous limb with an up and down 
stroke, he is enabled to scull himself along at various 
whales’ fool. 
