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abe
abf
abfa
abfb
abfc
abfd
abfda
abfe
abfea
abfeb
abff
abffa
abffb

~xperimental wind tunnel methods]
[Visualisation methods]
Using shadowgraph (3)
   Using photography. Photorecording (3)
       Drum camera (1)
       Photomultiplier (1)
       Television (1)
       Motion picture (2)
         High speed (1)
      Schlieren (2)
          Spa rk (1)
            Photomultiplier (1)
      Interferometry (2)
         Fringe shift (1)
         Interferential strioscopy (1)
      etc.

The full hierarchy provides twenty-nine classes.

        The first reduction, replacing the terms marked (1) by a see reference to their
immediate containing head (e. g. Drum camera see Photography), leaves eleven classes,
namely

a                      Experimental wind tunnel methods
ab Visualisation methods
aba Using smoke,vapours etc.
abb Using coatings, flows etc.
abc Using spectrum
abd  Using stroboscope
abeUsing shadowgraph
abf Using photography
abfd Motion pictur˘
abfeSchlieren
abff   Interferometry

         The second reduction, similarly replacing terms marked (2), which now include
 those originally marked (1), by see references to their containing heads, leaves eight
 classes, namely

a
• ab
 aba
 abb
 abc
 abd
 abe
 abf

          Experimental wind tunnel methods
            Visualisation methods
                                Using smoke,vapours etc.
                                Using coatings, flows etc,
                                Using spectrum
                                Using stroboscope
                                Using shadowgraph
                                Using photography

        The third reduction similarly replacing terms marked (3), leaves two classes,
namely
a                           Experimental wind tunnel methods
ab Visualisation methods

    In this way figures are obtainable to show the exact effect of moving, say, from