it that the two streams, although moving with the same velocity, are moving in different directions along the surface of separation; there would appear to be nothing contrary to hydrodynamic principles in this supposition; and the result would be a surface of discontinuity which might conceivably satisfy the condition.
The subject is one of very great difficulty, and it is impossible to do more than point out the more probable interpretations.
§ 191. Sectional Form.—The simple arc form of section employed as an illustration in § 188 is, qualitatively speaking, representative of that which may be considered essential, although the actual section more commonly resembles that given in Fig. 57, which may be regarded as typical.
We have seen that the consequence of an excessive "dip" on
Fig. 120.
the front edge is a loss of sustaining reaction, and it would appear that the trouble is not so much due to the excessive angle of dip but to the fact that the leading edge comes down too low; it is evident, therefore, that the leading edge, after being allotted the theoretical angle and position, as in Fig. 120 (indicated by the dotted line), should be curtailed somewhat in the manner shown.
In the wings of birds the elastic nature of the trailing portion probably acts as a considerable safeguard, for should the pressure reaction show any sign of falling off, the elasticity of the plumage will immediately rectify matters; it is at least impossible to get any sudden reversal as may happen when the aerofoil is a rigid structure.
In § 120 it was suggested that the form of section might be
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