FORM GIVEN TO THE SURFACE OF THE FLUID. 65 



agitation is thus extended to particles remote from the body. In certain cases 

 motion is thus communicated to particles before the body, so that when it reaches 

 them it neither finds them in a state of rest nor terminated by a horizontal plane. 

 This change of form must constitute an important element in the resistance ex- 

 perienced by the floating body. 



The form which a fluid assumes when disturbed by a body moving with a 

 velocity less than that of the wave, is very different from that which it takes 

 when the velocity of the body is greater than that of the wave. 



The phenomena attending velocities less than that of the wave, which are 

 most general and important, are the Great Anterior Wave of Displacement, the 

 Posterior Wave of Rejjlacement, and the Lateral Current. The secondary wave 

 of excessive displacement and the secondary wave of replacement, are pheno- 

 mena of a peculiar and accidental nature, resulting from the form of the disturb- 

 ing body. 



The great anterior wave of displacement is produced by the translation of 

 the fluid from the path of the solid — the mass of displaced fluid forms an eleva- 

 tion towards the anterior parts of the vessel, which is propagated continually for- 

 wards in the direction of the motion in the form of the wave, and with the velo- 

 city due to half the depth of the fluid. This anterior accumulation is constantly 

 maintained by the continual displacement of the moving body, and forms a smooth 

 well defined wave, extending many feet forward from the bow of the vessel, and 

 across the whole width of the channel. The rounded summit of this wave is 

 placed at low velocities considerably anterior to the stem of the vessel. At low 

 velocities also the wave is small, but the wave increases with the increase of the 

 velocity of the vessel, and at the same time the vessel is brought forward towards 

 the highest part of the wave. - ■ 



The lateral current of the fluid around the vessel from the stem towards the 

 stern is a phenomenon that always accompanies the anterior wave. The eleva- 

 tion of the fluid anterior to the solid by its introduction into the space occupied 

 by the anterior fluid, and the removal of the posterior part of the solid from the 

 space previously occupied by it, form an elevation and depression, of which the 

 inequality of the pressure determines a current with a given velocity in a direc- 

 tion opposite to that of the motion of the solid. 



The great posterior wave of replacement is totally different in the nature of its 

 generation and the law of its propagation from the anterior wave of displace- 

 ment, and ought not in any way to be confounded with it. It is of the nature 

 of an oscillatory wave, and frequently degenerates into a surge or breaking wave. 

 It is formed in the following way : The motion of the solid having sent forward 

 the particles of the fluid before it in the form of the anterior wave, there remains, 

 when the posterior part of the bod^^ is withdrawn from a given part of the chan- 



VOL. XIV. PART I. I 



