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 1s, 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, 2.¢., 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 
