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{|Communications were done by the instructor through a speaking tube
Speaking tube
A speaking tube or voicepipe is a device based on two cones connected by an air pipe through which speech can be transmitted over an extended distance. While its most common use was in intra-ship communications, the principle was also used in fine homes and offices of the 19th century, as well as...

 to the after-seated student. Communications back were agreed-upon gestures.

See also

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The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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