ACOUSTIC FIGURES. 253 



drical ones of different diameters. On the extremities of 

 these rods are fixed small quicksilvered glass beads, either 

 singly or in groups, so that when the instrument is 

 placed in the light of the sun or in that of a lamp, bright 

 images of the sun or candle are seen reflected on each 

 bead. If any of these rods is set a-vibrating, these 

 luminous images will form continuous and returning 

 curve lines in a state of constant variation, each different 

 rod giving curves of different characters. 



The melodion, an instrument of great power, embracing 

 five octaves, operates by means of the vibrations of 

 metallic rods of unequal lengths, fixed at one end and 

 free at the other.* A narrow and thin plate of copper is 

 screwed to the free extremity of each rod, and at right 

 angles to its length ; and its surface is covered with a 

 small piece of felt impregnated with rosin. This narrow 

 band is placed near the circumference of a revolving 

 cylinder, and by touching the key it is made to descend 

 till it touches the revolving cylinder, and gives out its 

 sound. The sweetness and power of this instrument are 

 unrivalled, and such is the character of its tones, that 

 persons of a nervous temperament are often entirely 

 overpowered by its effects. 



The vibrations of plates of metal or glass of various 

 forms exhibit a series of the most extraordinary phenomena 

 which are capable of being shown by very simple means. 

 These phenomena are displayed in an infinite variety of 

 regular figures assumed by sand, or fine lycopodium 

 powder, strewed over the surface of the glass plate. In 

 order to produce these figures, we must pinch or damp 

 the plate at one or more places, and when the sand is 

 strewed upon its surface it is thrown into vibrations by 

 drawing a fiddle-bow over different parts of its circum- 



* See Edinburgh Encyclopaedia, Art. SCIENCE, CURIOSITIES IN, 

 vol. xvii. p. 563. 



