430 



NA TURE 



\ April I, 1875 



That an undulation travelling alonij a median fin mast 

 act as a propsUer in a direction the reverse of that in 

 which the wave travels, is evident; because eich small 

 section of the fia can be easily reco^nisei to consist, as 

 long as it is in motion, of an inclined plane of which the 

 surface of impact against the water is at all times directed 

 backwards as well as laterally, just in the sime way that 

 in sculling from the back of a bait the propelling surface 

 of the oar is always similarly directed. 



This undulatory motion of the fia is produced by the 

 lateral movement, in a given constant order, of the spines 



which go to compose it ; the movement being at right 

 angles to the long axis of the body, and consequently at 

 right angles to the direction in which the fish travels. A 

 delicate memb-ane intervenes between each two spines, 

 which participates in their changes in position, and forms 

 the inclined planes above spoken of. 



Each spine is swollen at its base, where it articulates 

 with the corresponding internearal spine which is em- 

 bedded in the substance of the aiimal, and runs suffi- 

 ciently deeply to become situated between the spinous 

 processes of the two nearest vertebrae. An elongate fusi- 



of the boat constructed by Messrs. Elliott, with the undulating propeller de .cribed in the text. 



form muscle runs from each side of the swollen base of 

 the moveable spine, parallel to the spinous processes of 

 the adjacent vertebra:, to be fixed at its proximal or deeper 

 end to the body of the vertebra which is situated just 

 beneath it. By the action of the one or other of the pair of 

 muscles attached to each spine, the latter cm be moved 

 to the right or to the left of the body of the fish. A 

 similar couple of muscles acts on each of the elenents of 

 the dorsal fin, which is not co nplicated by any additional 

 machinery to produce the elegant movement observed 

 when it is in action during life ; this, therefore, must be 



dependent on the peculiarity in the ner*e-supply, with 

 which it is not as yet possible to associate any special 

 structural organisation. 



It is not difficult to imitate artificially this undulatory 

 fin of the above-mentioned fish. A series of rods hinged 

 near their middle on a single axis will evidently represent 

 at one end any movements given to them at the other. 

 Therefore, if they are made to come in contact at one extre- 

 mity with the side of a screw which is placed perpendicular 

 to their direction, and at the same time is provided with 

 projecting discs at right angles to its axis, one between 



Fig. 3.— The same buat l..ukej at fi^i,. u^.c.>. tl,^.- .p.^. 



every t\\ o rods, to keep them in place, the opposite tips 

 will form an undulating curve, just in the same way that 

 the ivory balls in the eccentric apparatus so frequently em- 

 ployed by lecturers on experimental physics, are made to 

 represent the undulations of the atoms of the luminiferous 

 aitherinthe production of light. Like this apparatus also, 

 if the screw be made to rotate, an undulation will travel 

 along the rods, which is exactly similar to that observed 

 in the fin of the Sea-horse. Such a piece of machinery, 

 driven by clockwork, ought theoretically to propel a boat 

 if properly placed. Mr. C. Becker, of the firm of Messrs. 

 Elliott and Co., has constructed such a boat, which is the 

 property of the Royal Institution (seen sideways in Fig. 2 

 and from below in Fig. 3.) Its speed is slow, as is 

 that of the fish ; in the former case this is accounted 

 for by the fac that the machinery is in this particular 



u. u.e . ... J . i^. ...Ills ua..i iilating propeller being seen. 



instance perhaps a little too heavy, at the same time that 

 the friction developed in its action is very considerable. 

 In the artificial fin there are just three complete undula- 

 tions with eight rods in each semi-undu'.ation, forty-eight in 

 all. Between the rods the membranous portion of the 

 fish's fin is represented by oil-silk. The rods and the 

 other portions of the driving gear are so arranged that the 

 former project, with their undulating ends and the oil- 

 silk, in the middle of the boat, along the line of the keel. 

 They form what may be termed a median ventral fin. 

 The undulations are very complete, the curves being true 

 semicircles. In the different species of Sea-horses and 

 Pipe-fish the number of spines in the dorsal fin differ, 

 being twenty or nineteen in Hippocampus antiqiiorum, 

 thirty-seven in a most eccentric looking species described 

 by Dr. Giinther/and named by him Phylhipt(?y.r egiics,z.nd 



