80 PHYSICS. 
formed on the rods, consisting of the points of rest formed by the individual 
molecules during their motion produced by the vibrations. These nodal 
lines form peculiar curves, which exhibit a certain similarity to a greatly 
elongated helix, forming a node at each revolution. The inner surface of a 
vibrating tube presents nodal lines similar to those of a rod. In prismatic 
rods the nodal lines are more complicated. 
’ f. Tongue Work and Reed Pipes. 
Any thin plate set into vibration by a current is called a tongue. Thus, 
in pl. 19, fig. 95, Wl is a tongue, which, by means of a small screw, is so 
attached to a plate that it can vibrate in the little aperture, nbed, 
without touching the edges. The plate may be of brass or zinc; the 
tongue, //, should be a very thin elastic slip of brass. The current must be 
directed against the free end of the tongue, //; sounding vibrations are thus 
produced by the alternate opening and closing of the aperture, whose 
length depends upon the number of vibrations of the tongue. The 
accordeon is a combination of several tongues, yielding the successive notes 
of the scale; these are placed upon a sound-board, and played upon by air 
from a bellows. Each tongue has its valve, which may be opened by a 
stop, and the air thus admitted to produce vibrations in the tongue. 
The tongue-work in an organ has a similar construction, although the 
attachment is somewhat different. Fig. 97 represents the arrangement on 
a large scale. The tongue-work consists of a pedal, p, in which is a hollow 
channel, which appears above as a round hole. This channel is closed by 
the plate 7, in whose opening is the tongue J, to be vibrated by the air 
passing through the channel. To tune the tongue its length must be 
changed, for which purpose there is a tuning-wire passing through the 
pedal, and by its two extremities pressing the tongue against the plate. 
The vibration of that part of the tongue between the plate and the wire is 
thus prevented. 
This tongue-work is combined with the pipe, ¢ (fig. 96), in such a 
manner, that the air entering through the pedal of the pipe presses against 
the tongue, setting it in vibration. It then escapes through an opening in 
the head, t’. When the pipe is used, by way of illustration, a glass plate 
lies before ab, to exhibit the action of the tongue. Sometimes, and gene- 
rally in organs, the tongue-work is placed in the pedal, and the tube is then 
directed upwards. 
The form of the tube gives character to the tone ; thus the trumpet works 
have tin tubes widening above, &c. In such tongue-work, however, the 
vibrations of the tongue depend upon the motion of the column vibrating 
in the long tube, and the tongue is more vibrated than if it made entirely 
independent vibrations. 
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