LOCOMOTION IN WATER 217 



Special Arrangement for Studying this Movement. — 

 The light should come from below ; in fact, the 

 arrangement shown in Fig. 51, Chap. V., answers very 

 well for these researches. The eel is silhouetted 

 against the luminous background, and the object-glass 

 of the apparatus is directed vertically downwards, or 

 else a silvered mirror, inclined obliquely at an angle of 

 45°, reflects the image of the fish towards the object- 

 glass, which is then set horizontally. 



Fig. 152 represents a series of photographs in which 

 the progression of the animal can be followed, as well 



Fig. 152.— Eel moving in a horizontal plane. The horizontal line oo enables the 

 reader to appreciate the degree of obliquity of the lines which join the veniral and 

 n<dal portions of the curves into which the body is thrown. The degree of velocity 

 of progression is expressed by the obliquity of the line aa. 



as the undulatory movements along its body. The 

 oblique lines indicate the propagation of these waves 

 in relation to the horizontal line oo with which the 

 head of the animal would be on a level in the entire 

 series were it not for the progress made. The line aa, 

 obliquely inclined, shows in each instance how far the 

 eel has advanced. This line is straight, and conse- 

 quently proves that the velocity is uniform. In 

 the fifth image, i.e. at the end of half a second, the eel 

 has advanced a distance equal to a quarter of its own 

 length, say about 0*075 metres, which corresponds 

 to a rate of 0*15 metres per second. Further, the 



