248 



Mr. C. Tomlinson. 



[Apr. 16, 



When Wheatstone reproduced Chladni's figures on square plates 

 ("Phil. Trans.," 1833, p. 593), lie did not notice the remarkable 

 figures produced by mixing a fine powder with the sand. This was 

 the less necessary because Faraday's memoir had been so recently pub- 

 lished, and its conclusion was so satisfactory, namely, that when a 

 plate is vibrating, currents are established in the air lying upon the 

 surface of the plate, which pass from the nodal lines towards the 

 centres of maximum vibration, and then proceeding outwards from 

 the plate to a greater or less distance, return towards the nodal 

 lines. 



With the exception of a very few elementary specimens on a small 

 scale, as given by Chladni and Faraday, I am not aware that this 

 class of figures has ever been adequately represented ; and yet these 

 figures are not only interesting in themselves, but are capable of being 

 exhibited on a scale fitted for the lecture table. For some of the 

 figures we may make use of a brass plate, 12 inches square, fixed on a 

 pillar by means of a central screw. The upper surface of the plate 

 should be blackened by means of nitrate of silver or chloride of 

 platinum solution, or some such contrivance, so as to leave a stain not 

 liable to crack or peel off. 



A thin layer of sand is to be dusted over the surface by means of a 

 pepper-box, and then a small bag of* thin linen or calico, containing- 

 the lycopodium powder, is to be shaken over the layer of sand. On 

 applying the bow to the edge of the plate, 3^ inches from the corner, 

 the note produced in my plate is Efc| in the fourth space. This pro- 

 duces fig. 1 with well-defined nodal lines and heaps of lycopodium 

 powder on the twelve centres of vibration. Under the continued 

 action of the bow each heap expands, swells up, and contracts with 

 great activity. If while this figure is at rest the bow be applied to the 

 centre of the edge, so as to produce the higher note Bb above the 

 lines in the treble clef, fig. 2 is produced, a figure of great beauty, 

 from the symmetrical arrangement of its parts and the delicate 

 variations of shading, which confer upon it an artistic character. 



Iron filings were sifted over the plate, and on sounding the note 

 that leads to the formation of fig. 1, the finest particles of iron were 

 transferred to the centres of vibration, while the coarser remained at 

 the nodes. 



Lycopodium alone was dusted over the plate, when the powder 

 became divided into two portions, one remaining on the nodal lines, 

 now very much broader than those formed by sand, while a second 

 portion assumed active functions on the vibrating centres. Lord 

 Justice Fry, F.R.S., to whom I showed this experiment, suggested 

 that the winnowing process which separated the finer from the coarser 

 particles in the former experiments might also apply here. He col- 

 lected specimens from the nodes and from the vibrating centres, but 



