424 NATURAL SCIENCE. JUNE, 
mud-particles. When the water is disturbed by the passage of waves, 
the sphere becomes an ellipsoid, but it does not mix with the surrounding 
water. The longest axis of the ellipsoid is vertical when the mass is 
under (or within) the crest of a wave, and its shortest axis is hori- 
zontal and parallel to the direction in which the waves are travelling 
—this direction we will call longitudinal. The transverse axis of 
the ellipsoid, that is, the axis parallel to the crest of the wave, 
remains unchanged throughout, and therefore equal to the diameter 
of the spherical mass we started with. As thisremains unaltered, we 
may leave it out from further consideration. 
As the mass we are considering falls into the trough behind, that 
is, as the wave recedes from it, it comes to be elongated in a series of 
new directions. At first sight, the mass appears to rotate in the direc- 
tion in which each of its parts revolves, so that the upper end of the long 
axis comes to be inclined forwards till when the bottom of the trough 
is reached this long axis is horizontal and longitudinal. The mass, 
however, does not really rotate: the point which was uppermost when 
the mass was at the crest of the wave is uppermost still when in the 
trough. The form of the ellipsoid rotates, while its substance does 
not. The long axis is vertical every time the mass is at the crest of 
a wave. In passing from one wave-crest to the next the form (not the 
substance) rotates through half a revolution. During the same period 
each particle performs a complete civculay journey. The relative move- 
ment of any two particles in the mass is one of oscillation. Take, for 
instance, the uppermost and the lowermost particles of the spherical 
mass with which we started. When the water is still, one is vertically 
above the other. During the movement the upper one (A) moves in 
a circle, and the lower one (B) moves in a smaller circle. A is there- 
fore above B when both are under the crest, and again when both are 
under the bottom of the trough. At every moment A and B are 
moving in the same direction, but A always faster than B. At the 
crest both are moving forwards, A therefore comes to be slightly in 
front of B, but as both turn to move backwards, A comes to be above 
B again in the trough and then comes to be behind it. If we take 
two other particles a similar result is reached, but with a difference. 
If the two particles are not in the same vertical transverse plane when 
at rest, then they will not at any moment be moving in the same 
direction. Suppose C and D be particles at the two ends of the 
“longitudinal” diameter of the sphere in still water, C being in front, 
D behind. Then as the waves advance D will always move some- 
what before C, so that when the middle of the mass is under the crest 
D will already have begun to move downwards, while C is still 
moving upwards. At this stage C and D will also be nearer together 
than at any other. In the trough these will be further apart than at 
any other stage, and while both are moving backwards D will already 
be moving backwards and upwards, while C is still moving backwards 
and downwards. 
