RESPECTING SOUND AND LIGHT. 



549 



mities of that part. An observation of Dr. 

 W allis seems to have passed unnoticed by 

 later writers on harmonics. He says, that 

 if the stringofaviohii be struck in the mid- 

 dle, or at any other aliquot part, it will give 

 either no sound at all, or a very obscure one. 

 This is true, not of inflection, but of the 

 motion communicated by a bow ; and may 

 be explained from the circumstance of the 

 successive impulses, reflected from the fixed 

 poitits at each end, destroying each other : 

 an explanation nearly analogous to some ob- 

 servations of Dr. Matthew Young on the mo- 

 tion of chords. When the bow is apphed not 

 exactly at the aliquot point, but very near it, 

 ihe correspondingharmontc is extremely loud ; 

 / and tli€ fuudamentat note, especially in the 

 lowestharnionics, scarcely audible : thechord 

 assumes the appearance, at the aliquot points, 

 of as many lucid lines as correspond to the 

 number of the harmonic, more nearly ap- 

 proachmg to each other as the bow ap- 

 proaches more nearly to the point. (Plate 5. 

 Fig. 54.) According to the various modes of 

 applying the bow, an immense variety of 

 figures of the orbits are produced, (Fig. 48.) 

 more than enough to account for all the dif- 

 ference of torte in different performers. In 

 experiments of this kind, a series of harmo- 

 nics is frequently heard in drawing the bow 

 across the siime part of the chorti : these are 

 produced by the bow ; they are however not 

 proportionate to the whole length of the 

 bow, but depend on the capability of the 

 portion oftiie bowstring, intercepted between 

 its end and the chord, of performing its vi- 

 brations in times which are- aliquot parts of 

 the vibration of the chord : hence we may 

 perhaps infer, that the bow takes effecton the 

 chord but at one instant, or for a very short 

 time, during each fundamental vibration. la 



these experiments, the bow was strung with 

 the second string of a violin. 



XIV. Of the Vibratiom of Rods and Plates. 



Some experiments were made, with the as- 

 sistance of a most excellent practical musi- 

 cian, on, the various notes produced by a 

 glass tube, an iron rod, and a wooden ruler;, 

 and, in a case where the tube was as much at 

 liberty as possible, all the harmonics corre- 

 sponding to the nmnbers from 1 to 13, were 

 distinctly observed ; several of them at the 

 same time, and others by means of difTerent 

 blows. Tliis result seems to differ from the 

 calculations of Euler and Count Iliccati, con- 

 firmed as they are by the repeated experi- 

 ments of Professor Chladni ; it is not there- 

 fore brought forward as sufficiently contro- 

 verting those calculations, but as showing 

 the propriety of an inquiry into the sources 

 of error in such experiments. Scarcely 

 any note could ever be heard when a 

 rod was loosely held at its extremity ; noi 

 when it was held in the middle, and struck 

 one seventh of the length from one end. The 

 very ingenious method of Professor Chladni,. 

 of observing the vibrations of plates by 

 strewing fine sand over them, and discover- 

 ing the quiescent hues by the figures into 

 which it is thrown, has hitherto been little 

 known in this country: his treatise on the 

 phenomena is s<j complete, that no other 

 experiments of the kind were thought neces- 

 sary. Glass vessels of various descriptions,, 

 whether made to sound by percussion or 

 friction, were fbun Jto be entirely free from the 

 usual harmonic notes ; and this observation 

 coincides with the experiments of Chladni.. 



XV. Of the Human Voice. 

 Tlie human voice, which was the object 



