SOUND 273 



plate held at the centre may be arranged so that one quad- 

 rant of it projects into the field and is projected, (b) A 

 smaller plate may be clamped in the centre, at the end of an 

 arm projecting from outside the field, (c) The figures may 

 be formed apart and then projected whole, (d) A plate held 

 at one edge may be vibrated in the field, by a bow, or by a 

 string attached to it and rubbed with resined fingers, or by a 

 resined string drawn through an aperture in the centre. 

 (e) Or vibration may be communicated to it from some other 

 vibrating apparatus in any of the recognised ways. (/) Or 

 on one corner of a large plate may be cemented a ring of 

 rubber, within which water is poured an eighth of an inch 

 deep a favourite experiment of Professor Morton. When 

 the plate is bowed, a beautiful network of waves will be pro- 

 jected. 



150. Columns of Air. Kundt's dust- figures, showing the 

 nodes and segments of air in a glass tube, may be formed in any 

 way and then projected, or the tube may be placed in the field 

 of the vertical attachment and the dust shown in vibration. 

 A very convenient method is to plug one end of a tuba, and 

 insert a short whistle of appropriate pitch into a cork in the 

 other. Any of Mach's experiments may be projected in the 

 same way. Square tubes cemented together of flat glass plates 

 are worth the trouble of construction, the figures being so 

 much more accurately focussed upon the flat glass. 



The projection of tracings upon smoked glass from tuning- 

 forks, membranes, &c., is so obvious and simple as to require 

 no more than mention. 



