fluid. When a hole is made in the plane, the air flows through from the front to the rear under the influence of the difference of pressure between its two faces. The stream of air finding its way through the perforation carries with it an amount of momentum per second, equal to the force of which the plane is relieved. If there were a conduit to carry this efflux air away without interfering with the dead-water,
Fig. 91. then the plane would show itself relieved of pressure to the extent of the area taken away together with an addendum due to the fall of pressure adjacent to the perforation, in accordance with the well-known principles of efflux theory. But there is no conduit to carry away the efflux air, which consequently passes into and becomes mixed up with the turbulent dead-water; and the efflux air carries with it its momentum, which is communicated to the dead-water, and momentum communicated to the dead-water appears as negative pressure on the rear face of the plane, since it is the
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