constrained to move in a congruent manner. Thus if one layer be supposed to strike the plane and follow its surface, the next layer will be in turn deflected and move parallel to the first, and so on. If the particles of fluid were artificially constrained so as to be unable to undergo any change of velocity along the axis of flight, or to spread laterally, this influence would be transmitted from layer to layer with undiminished amplitude, or in the case of an elastic fluid until the initial displacement had been absorbed by compression.
Fig. 107. If we suppose the artificial constraint to be removed, then the amplitude rapidly diminishes as we get further from the plane owing to the longitudinal motions of the fluid particles; this may be regarded as a leakage of the fluid round the plane from the compression to the rarefaction side. (Compare Chap. IV., § 109.)
Now the facility with which the air or fluid can escape round the plane from one side to the other is evidently, for small angles at any rate, independent of the angle and dependent only on the size and shape of the plane, and for planes of elongate form it evidently depends largely upon the smaller dimension and to a less extent upon the greater. Thus in the case of a plane in pterygoid aspect the thickness of the layer affected by the passage of the plane will depend upon the dimensions of the
