considerations. The term surface of discontinuity may be looked upon as an abstraction of that which is essential in a somewhat complex phenomenon.
§ 21. Experimental Demonstration of Kinetic Dscontinuity.—The reality and importance of the discontinuous type of motion can be demonstrated conclusively by experiment.
In Fig. 8, a, b, c, is a hollow spherical globe in which d is a tube arranged to project in the manner shown. An ordinary lamp globe and chimney will be found to answer the purpose
Fig. 8.
the former having one of its apertures closed by a paper disc. The whole is carefully filled with smoke and then moved through the air in a direction from right to left, the relative direction of the air being indicated by the arrow.
It will be found that the air will enter the tube and displace the smoke through the annular aperture. The issuing smoke follows the surface of the sphere in the most approved manner as far as the "equator," but then passes away at a tangent, the stratum of discontinuity, the dead-water region, and the turbulent character of motion, being all clearly manifest. The discontinuity, as may have been anticipated, does not appear as a clean-cut surface; it is marked almost from the commencement, as indicated in the figure, by eddy motion; but when we remember
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