ON WAVES. 383 



to the observer, eight miles off, the instant at which the bell was struck. 

 This sound was distinctly heard by a sort of ear-trumpet lowered in the 

 water at the other end, and so the observations made. 



The mean time occupied in propagating the sound from one station to 

 the other as thus determined, was nine seconds and a half, or more precisely 

 9-4 seconds, giving for the velocity of sound by direct experiment 



c=i?MZ=1435 metres, 

 9-4. 



the actual velocity of the sound wave thus being found to differ from the 

 tlieoretical by not three metres per second. 



The velocity of transmission of the wave of the fourth order in water is 

 therefore in English measure about 1580 yards per second, being about one- 

 half more rapid than the velocity of sound through the atmosphere. 



DESCRIPTION OF THE PLATES. 



Plate XL VII. 



Genesis and Mechanism of the Wave of Translation. — Order I. 



Fig. 1. Genesis by impulsion. — A X is the bottom of a long rectangular chan- 

 nel filled with water to a uniform depth ; P a thin plate inserted vertically 

 in the fluid and fitting the internal surface of the channel. It is moved 

 forward from A towards X through the successive positions Pj, Po, P3, P4J 

 P5, and heaping up the water before it generates a wave of the first 

 order W^, W5, which is transmitted along the channel as at W5, Wg to 

 W3, &c., being transmitted with uniform velocity as a great solitary wave, 

 and leaving the water behind it in repose at the original level. 



Fig. 2. Genesis by a column of fluid. — In the same channel the moveable 

 plate P7 is fixed so as with the end and sides of the channel to form reser- 

 voir A G P4, containing a column of water G W, raised above the surface 

 of repose of the water in the channel. P^ is suddenly raised as at P„ and 

 Pg ; the column descends, presses forward the column anterior to P, and 

 raises the surface, generating a wave of translation, which is transmitted 

 along the channel as before. After genesis the volume g^ reposes on the 

 level g<a the water in the channel having been translated forwards from P 

 to kk ; every particle of water in the channel has during the transmission 

 of the wave been translated towards X through a horizontal distance equal 

 toP^. 



Fig. 3. Genesis by protrusion of a solid. — Lj is a solid suspended at the end 

 of the channel, its inferior surface slightly immersed in the fluid. It is 

 suddenly detached, descends, displaces the adjacent fluid, and generates a 

 wave of translation as in the foregoing methods. 



Fig. 4 exhibits the phsenomena of genesis, transmission and regenesis, or 

 reflexion of the wave of translation. 



Fig. 5 exhibits in four diagrams the motions of individual wave particles 

 during wave transmission. The first diagram represents by arrows the 

 simultaneous motions of the particles in different portions of the same wave 

 at successive points in its length. At the front of the wave the particles 

 a, c, e, g, taken at equal depths below the surface, are at rest. The wave- 

 length is divided into ten equal parts : at the first the motion is chiefly up- 

 wards, and very slightly forwards ; at the second, less upwards and more 



