336 CONDUCTION OF SOUND. 



with both hands close to their faces, stretching the twine j 

 tightly as possible without breaking it or the thin boards. To hoi 

 them long is, however, tiring, and the tapping could not be doi 

 without the assistance of a third person ; it is therefore more co] 

 venient to fix the boards otherwise. This may be done by using tl 

 opposite windows of two houses, 100 or 150 m distant from 01 

 another. Each box must then be supported upon a board (fig. 21 

 A), about O m> 5 or O m '6 long, and a little wider than the box ; eac 

 board has a square or round hole cut through it, 6 or 8 cra wid 

 through which the cord passes freely. The boxes are kept in poh 

 tion by a few pins driven close to them into their supports. If tl 

 sash of the window is raised, it will be easy to adjust the board 

 that it is held by the sash and the frame of the window, if the co 

 is stretched. The opposite board may be similarly supported, b 

 as the thread cannot be tied until afterwards, it has not the requir 

 tightness when fixed, and the board must be pulled back. Tl 

 may be done by boring holes at the four corners of the board, fixi 

 cords to them and pulling the cords tight, and attaching them > 

 a hook in the wall, or the handle of the door. The board whi i 

 carries the sounding-box may then be left quite free, or it may 

 supported by the back of a chair. The cord which transmits 1 ? 

 sound must be perfectly free throughout its length. Anotl 

 mode of supporting the boxes is shown in fig. 211, C. Two sir! 

 ladders are erected in the open air, and kept steady by stretcl 1 

 ropes, which are either fastened to trees or to pegs driven into L- 

 ground. The boards with the sounding-boxes are supported by 1 * 

 nails driven into the ladder, and the communicating cord pas - 

 between two of the rounds. 



Iron wire is better than twine, and allows of experiments be 

 made over a greater distance, but as it sinks considerably in < 

 middle in consequence of its weight, it requires somewhat 1 

 points of support. It may, however, be supported in the mi< 

 in the following manner. Two rather long poles are driven slani 'i: 

 into the ground, so as to cross near their top; where they cross 

 piece of twine is tied to them, and the wire is supported by '' 

 twine, so as to pass between the poles one or two decimetres 

 the crossing. The stretching of a wire which is several hum ( 

 metres long, is rather troublesome and requires several pers 

 great care must be taken that the wire is pretty nearly straigl v 

 the outset, and especially that it has no kinks, or it will break v ' 

 tightened. It will generally be necessary to put together a " 

 of such a length as is required, by joining several pieces : the P* 

 should be annealed and twisted together in the manner shown ii - 

 211, B. 



