May, 1893. BIOLOGICAL THEORIES. 351 



partly up-and-down but partly in other directions. The details of 

 this wave-movement will have to be considered in a future article, 

 and it is only necessary here to recognise that the wave consists of an 

 obvious movement ; that its velocity is small enough to enable the 

 progress of the wave to be watched ; that it is transmitted only 

 through a superficial layer of the water, the thickness of which 

 varies with the intensity of the disturbance; and, finally, that its pro- 

 pagation depends upon gravitation and surface-tension. 



The sound-waves agree in number with the surface-waves, and 

 there the agreement practically ends. These waves are waves of 

 compression rather than of movement ; their rate of transmission is 

 about a mile a second ; their propagation depends entirely upon the 

 elasticity of the water, that is, the property by virtue of which the 

 water resists voluminal compression, and regains its original volume 

 after compression ; the movement of the water involved in the trans- 

 mission of such waves, even when the sound is of explosive intensity, 

 is far too minute to be seen ; this minute movement is, moreover, 

 strictly to and fro in the direction of propagation of the wave ; the 

 wave is transmitted not through a superficial layer only, but in all 

 directions through the water. 



If the aquarium be divided into two parts by a water-tight 

 partition of wood, glass, iron, or other solid, the surface-waves will be 

 stopped by this partition, while the sound-waves will pass unimpeded 

 through it. 



Sound-waves in air differ from those in water in the amount of 

 movement of the particles transmitting the wave. Here, also, the 

 wave is one of pressure rather than of movement, though movement 

 ten thousand times as great as that in an aquatic sound-wave is 

 involved. 



Some idea of the minuteness of these movements may be 

 obtained from the determination made by Lord Rayleigh and 

 recorded in Ganot's " Physics." The note experimented upon was 

 f^ produced by an organ pipe. The intensity was such that the note 

 was audible at a distance of more than half-a-mile. The movement 

 of each air-particle was less than one two-hundred-and-fifty-millionth 

 of an inch, i.e., little more than one five-hundred-millionth of a wave- 

 length. 



Water is only one ten-thousandth part so compressible as air, and 

 a like variation in pressure would, therefore, only involve in water a 

 movement of each particle of about one five-billionth of a wave- 

 length. 



Whether Hensen's experiments were performed in a vessel 

 sufficiently large for the accommodation of a series of waves each 

 five billion times the length of the observed movements of the hairs 

 or not, I cannot say, — the Atlantic Ocean would be a vessel of dimen- 

 sions suitable for the experiment. What the notes were to which the 

 hairs "responded" I do not know — suppose, however, the lowest 



