BORELLI ON SWIMMING. 387 



stern, and turned and vibrated here and there, glide upon the surface 

 of the water in a direct and rapid motion, without the aid of lateral 

 oars. And fishes, in like manner, are not impelled by the rowing 

 of the side fins, as we have already said ; and we observe, that as often 

 as their tail is vibrated, they pass with the greatest velocity through 

 the water ; and, on the other hand, when the tail is not moved in the 

 slightest degree, we observe they continue in the same situation. 

 Therefore the contortion and vibration of the tail is the true cause of 

 their motion, as the vibration of that single oar in the stern of a boat is 

 the cause of its direct motion. 



But the mode in which both operations are effected, is this. The 

 single oar, whilst it obliquely impels the posterior water resisting its 

 support, necessarily forces the boat forwards, although in a zig-zag 

 course, by deviating from a straight track. But, because such deviation 

 is suddenly corrected, either by a contrary movement, or by a firm 

 retention of the oar in an oblique position, by performing the office of 

 a rudder, it happens, that those momentary deviations are not perceived, 

 and the direct motion is alone conspicuous. . 



Now let us consider the configuration and motion of the fish, abc. 

 Whilst the head, a, with the belly, b, of the fish, directly constitutes 

 its remaining mediety, bfc, it bends and vibrates laterally, violently 

 lashing the water on account of the lubricate connection of the spinal 

 vertebrae, which like strong bows, may be easily contorted and turned 

 to the right and left. The extreme production of the fish has, besides, 

 an ample flexible tail, de, composed of cartilaginous small rods, which 

 are invested by a subtle membrane, as the feet of geese, and may be 

 contracted and dilated in precisely the same way. Again, at the 

 supreme edge of the back, and at the lowest edge of the belly, are also 

 consimilar cartilaginous fins, which are bent, contracted and expanded 

 just as the tail. 



But the order of the motion is this. The fish, abc, being extended 

 in a straight line, it begins its motions by extending the tail, bfc, to- 

 wards the right side, g, which flexion is executed with such rule, that 

 the moved part, 6c, whilst it is turned about the centre, b, does not 

 retain the precise rectitude like the rays of a circle, but is incurved 

 with a two fold sinuosity about b, by moving forwards the side f, 

 towards the right side g, and about f, and by retroceding with the ex- 

 treme tail, d, towards the left side ; and such is the first motion, not 

 the rowing, but somewhat anticipation of the tail, similar to the antici- 

 pation of the feet of a frog swimming, like which the palmary fins of 



