only basis on which it is possible to account for the phenomenon of fluid resistance as experimentally known. Beyond this there are many examples and illustrations which are of especial interest, considered either as proofs of the theory itself or in relation to their actual consequence or utility.
A useful application of the principle is found in the screen employed on fast steamships to protect the navigating officer, and frequently the "watch," from the rush of air, without
Fig. 20.
obstructing the field of vision. This is illustrated diagrammatically in Fig. 20, in which it will be seen that the live stream is carried clear over the sailor's head, the latter being protected by the surface of discontinuity. A similar device is frequently adopted in connection with the dashboard of a motor car.
Evidence of the most striking kind of the existence of a surface of discontinuity is sometimes met with in the growth of trees in the immediate vicinity of the edge of a cliff (Fig. 21). It may be seen that the form of the surface is clearly delineated, the
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