166 NODAL LINES IN AIR. [SKCT. xvn. 



cates undulations, all the phenomena of vibrating plates may 

 be exhibited by sand strewed on paper or parchment, stretched 

 over a harmonica glass or large bell-shaped tumbler. In order 

 to give due tension to the paper or vellum, it must be wetted, 

 stretched over the glass, gummed round the edges, allowed to 

 dry, and varnished over, to prevent changes in its tension from 

 the humidity of the atmosphere. If a circular disc of glass 

 be held concentrically over this apparatus, with its plane 

 parallel to the surface of the paper, and set in vibration by 

 drawing a bow across its edge, so as to make sand on its surface 

 take any of Chladni's figures, the sand on the paper will 

 assume the very same form, in consequence of the vibrations 

 of the disc being communicated to the paper by the air. 

 When the disc is removed slowly in a horizontal direction, 

 the forms on the paper will correspond with those on the 

 disc, till the distance is too great for the air to convey the 

 vibrations. If the disc while vibrating be gradually more 

 and more inclined to the horizon, the figures on the paper 

 will vary by degrees ; and, when the vibrating disc is per- 

 pendicular to the horizon, the sand on the paper will form 

 into straight lines parallel to the surface of the disc, by 

 creeping along it instead of dancing up and down. If the 

 disc be made to turn round its vertical diameter while 

 vibrating, the nodal lines on the paper will revolve, and 

 exactly follow the motion of the disc. It appears, from this 

 experiment, that the motions of the aerial molecules in every 

 part of a spherical wave, propagated from a vibrating body 

 as a centre, are parallel to each other, and not divergent like 

 the radii of a circle. When a slow air is played on a flute 

 near this apparatus, each note calls up a particular form in 

 the sand, which the next note effaces, to establish its own. 

 The motion of the sand will even detect sounds that are in- 

 audible. By the vibrations of sand on a drum-head the be- 

 sieged have discovered the direction in which a counter-mine 

 was working. M. Savart, who made these beautiful experi- 

 ments, employed this apparatus to discover nodal lines in 



