MOTION. 



435 



ffect of the stroke. In the back stroke, the 

 thigh (d, fig. 229) moves first, describing an 

 arc of a circle about their iliac extremity, from 

 d to d' and d" successively. During this move- 

 ment the legs e and tarsi _/" are flexed pas- 

 sively, and carried forwards to e',J~', so as to 

 act with little effect on the water. The two 

 thighs d" d" begin to approach their greatest 

 flexion forwards, the legs e f c' and the phalanges 

 f J'" extend in succession, so that the water 

 opposes resistance to only one of them at a 

 time. The tarsi (_/") having been completely 

 extended outwards, the two legs are suddenly 

 carried backwards, pressing on the water, with 

 the entire plane of the tarsi, and as the silky 

 fringes which cover the phalanges are extended 

 at the same instant, their surface is considerably 

 augmented. The other parts of the two legs press- 

 ing upon the tarsi, as in the walk or the leap, 

 projects the body forward in the direction of 

 its axis. In walking, the two members of the 

 same pair act alternately, in order to serve, 

 each in its turn, as a support to the centre 

 of gravity. In swimming, on the contrary, the 

 body being supported by the water, the two 

 members move simultaneously, in order to give 

 the greater impulse, and it is in this respect 

 that swimming differs essentially from walking, 

 and more nearly approaches leaping. The 

 middle legs of the Dytiscus act in a manner 

 similar to the posterior, but being shorter 

 and weaker, contribute little towards accele- 

 rating the movements of the animal ; the an- 

 terior pair appear to be used chiefly for the 

 purpose of altering the direction of its motion. 

 The motions of insects in the water may be 

 thus explained : let a b (Jig. 230,) be the axis 

 of the body passing through the centre of gravity 

 o ; and let c o be the excess of the specific gra- 

 vity of the water over that of the insect, acting 

 in a vertical direction upwards ; and do the re- 

 sistance of the water to the insect moving in a 

 direction oblique to the axis of the body; this 



Fig. 230. 



tatter force is decomposed into two forces, one 

 in d g parallel, and the other in d e or g o 

 perpendicular to the body, the former of which 

 is lost, and the latter forces it obliquely down- 



wards and backwards ; this force being com- 

 bined with the force of the water c o produces 

 a resultant in h o, opposite to d o, which is 

 (he force by which the insect ascends passively. 

 On the other hand, when the insect moves ho- 

 rizontally with its axis inclined in a b, and 

 its centre of gravity in o, fjig. 231,) let its 



Fig. 231. 



A jiywe from Straus- Dur<okfieim, to Hhtitrate the 

 movements of insects szomimintf horizontally. 



velocity be represented by h o, so that the force 

 in this direction may be the resultant of the 

 forces of the locomotive organs, of the water 

 c o, and of the resistance which it opposes to 

 the motion of the body in the direction of d o, 

 opposite to h o. The resistance of the liquid 

 in d o, acting obliquely upon the plane of the 

 body, a b, (which is known from the velocity 

 of the body and the inclination of its axis to 

 the horizontal plane,) may be decomposed into 

 two forces, one in c/ g or e o parallel, and the 

 other in d e or g o perpendicular to the body ; 

 the former is ineffective, and the latter tends 

 to force the body backwards and downwards. 

 The force g o, being combined with that of 

 the water c o, produces the resultant i o, 

 which, with the force of the oars, must give 

 the two components of h o. Completing the 

 parallelogram of forces, of which h o is the 

 diagonal, we find that the legs must generate 

 a force represented in magnitude and direction 

 by J' o, and it is in the direction of this com- 

 ponent, or, more correctly, in a line parallel 

 to it, that the centre of force of the feet ought 

 to act, which is in fact the case. 



The Hydrophilus has nearly the same form 

 as the Dytiscus, but is not quite so well or- 

 ganized for swimming. The Gyrinus also, as 

 well as the HydrophiTus, swims in conformity 

 with the same principles as the Dytiscus. The 

 Nepa, being illorganized for swimming, usually 

 walks in the water. The Hydrometra being so 

 light, and having a tittle globule of air attached 

 to its feet, has the power of swimming on the 

 water without sinking. The Notoncctu, ni which 

 the centre of gravity lies above the centre of 



2 F 2 



