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104                    HAND-BOOK OF ACOUSTICS.
Again, from what has been said above, it will be seen, that at the centre, where the node occurs, the air is alternately compressed and rarefied; compressed, when two condensations meet, and rarefied, when two rarefactions meet. This can also be experimentally verified; for if the pipe were pierced at the centre and the hole covered air-tight by a piece of sheet india-rubber, this latter being acted upon by the condensations and rarefactions, would be alternately pressed outwards and inwards. The organ pipe (fig. 55)
Fig. 55.
has been thus pierced at the centre B, and also at A and 0, and the membranes covered by three little capsules (a section of each of which on an enlarged scale is shown at the left of the figure), from the cavities of which proceed three little gas jets, the gas being supplied by the three bent tubes which come from the hollow chamber P, which again is supplied by the tube S. Now on blow