1831.1 On a Peculiar Class of Acoustical Figures. 321 



to vibrate, the lycopodium collects in three divisions ; that 

 between the nodal lines does not proceed at once into a line 

 equidistant from the nodal lines and parallel to them, but 

 advances from the edges of the plate towards the middle by 

 paths, which are a little curved and Fig 6 



oblique to the edges where they occur 



near the nodal lines, but are almost 



,. i i i_ j *!, 



perpendicular to it elsewhere, and the 



powder gradually forms a line along the middle of the plate ; 

 it is only by continuing the experiment for some time that it 

 gathers up into a heap or cloud equidistant from the nodal 

 lines. But upon fixing card walls upon Fi ^ 



this plate, as in fig. 7, the course of the 

 powder within the cards was directly 

 parallel to them and to the edge, instead 

 of being perpendicular, and also directly towards the centre 

 of oscillation. To prove that it was not as a weight that 

 the card acted, but as an obstacle to the currents of air 

 formed, it was not moved from its place, but bent flat down 

 outwards, and then the fine powder resumed the courses it 

 took upon the plate when without the cards. Upon raising 

 the cards the first effect was reproduced. 



19. The lycopodium sprinkled over the extremities of such a 

 plate proceeds towards places equidistant from the sides and 

 near the ends, as at a, fig. 8 ; but Fig. 8. 



on cementing a piece of paper to the 



OX VI 



edge, so as to form a wall about one 



quarter or one third of an inch high, 



b, the powder immediately moved up to it, and retained this 



new place. In a longer narrow plate, similarly arranged, the 



powder could be made to pass to either edge, or to the 



middle, according as paper interceptors to the currents of air 



were applied. 



20. Plates of tin, four or five inches long, and from an inch 

 to two inches wide, fixed firmly at one end in a horizontal 

 position, and vibrated by applying the fingers, show the pro- 

 gress of the air and the light powders well. The vibrations 

 are of comparatively enormous extent, and the appearances 

 are consequently more instructive. 



21. If a tuning-fork be vibrated, then held horizontally 



