ACOUSTICS AND GRAVITATION. 



51 



The results with the simple tubes in question were much the same as those 

 obtained with organ-pipes and need no further comment. With the open 

 pipes, where the octave vanishes, a wave-length much longer than the funda- 

 mental is often seen, probably due to the blower in some way. 



46. Interference. This experiment succeeded beautifully with the strip 

 ss' of the blower placed between two coaxial pipes P and P' (fig. 74), each 

 about 10 cm. long and 2 cm. in diameter, for instance, and closed at the outer 

 end. Either pipe alone sounds vigorously when actuated by the blower. 

 With the two together there is a mere siffling, the wave running from end 

 to end of the (virtually) double closed pipe PP'. Nevertheless, there is abundant 

 room at mm for the escape of sound; indeed, one pipe, P for instance, may even 

 be placed at right angles to the other, leaving a wide open space, and still almost 

 the whole energy of one pipe is alternatingly captured by the other. The 

 avidity with which one pipe absorbs the vibrations of the other is an excellent 

 illustration of the reversal of spectrum lines. The nodes here are respectively 

 dense and rare, i. e., always opposite in the two pipes; hence the interference. 

 In the cross-pipe above, the nodes were necessarily identical in sign, and, 

 therefore, gave marked response. The same will be true if the pipes PP' are 

 each open at the further end. 



76 



Another form of the blower is given in figures 75 and 76, where pp r is a short 

 end of square-sectioned brass pipe, closed at one end p' and provided with a 

 tubulure P at the other. One of the edges of the brass pipe has been filed 

 down until a fine rift cc' can be cut into it with a thin blade. As before, 

 gg' are the guides of an adjustable strip ss', against which the lamella from 

 cc' plays. 



If tubes T of large diameter (5 cm., 10 cm., and more) are to be excited, the 

 strip ss' should be wide as shown, so that it may completely close the end of the 

 organ- tube, except the mouth at cc'. Brass and pasteboard tubes in this case 

 respond strongly, provided the distance sc has been properly set. For very 

 wide mouths an influx pipe at P' normal to pp f is often advantageous and in 

 this case also the apparatus may be doubled, as shown in figure 77. Here there 

 are two rifts c and c' and strips s and s' opposite each other on the common 

 guides gg', P being the influx pipe. The apparatus is placed flat upon the 

 pipe or object T to be energized with c or c' regulated. A tumbler of elliptic 

 section is interesting; when gg' passes by rotation from the major to the 

 minor axis, the pitch rises continuously about a tone. Mouths 5 to 10 cm. 



