94 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



(438.) 

 In bodies 

 possessing 

 unequal 

 elasticity. 



(439.) 

 Chaldni on 

 meteoric 

 bodies. 



(440.) 

 Young 

 Robison. 



(441.) 

 Savart. 



Propaga- 

 tion of 

 sound in 

 masses of 

 air, &c. 



more recently, Savart found the means of preserving 

 by transferring them to sheets of gummed paper. 

 There are few experiments in physics of a more 

 striking character, or which so plainly reveal minute 

 and complex motions so small and so rapid as to 

 he difficultly appreciated otherwise. Mr Faraday 

 and Mr Wheatstone have pursued the same enquiry 

 experimentally, and the latter has satisfactorily de- 

 duced the figures of Chladni's square plates from the 

 mechanical superposition of simple modes of vibra- 

 tion which are symmetrical and isochronous. (Phil. 

 Trans. 1833.) By so doing he has succeeded better 

 than the mathematicians, whose results on this sub- 

 ject have been very little practical. 



Chladni was the first to make experiments on the 

 vibrations of bodies whose elasticity varies in dif- 

 ferent directions. Thus he cut plates out of different 

 kinds of wood, and found the nodal curves unsym- 

 metrical in different directions depending on the 

 course of the fibres. The experiments were naturally 

 afterwards employed to illustrate the theory of ellip- 

 soidal waves on the undulatory hypothesis of Optics. 



The experiments of Chladni procured for him the 

 especial notice of Napoleon, by whose wish one of 

 his works was translated into French. He died in 

 1827, and besides his acoustical discoveries, he will 

 be remembered by the sagacity and boldness with 

 which he maintained the popular opinion of the 

 fall of heavy meteors from the sky, contrary to the 

 prevalent scepticism of philosophers. Chaldni's suc- 

 cess in establishing this important fact in natural 

 history is due, like his other physical inductions, to 

 the constancy and simple-mindedness with which he 

 attached himself to a strictly definite enquiry. 



We must not here enlarge upon the ingenious and 

 important investigations of Dr Thomas Young con- 

 nected with acoustics. Being chiefly connected with 

 his admirable Theory of Light, they will be noticed 

 in the chapter on Optics. The peculiarly practical 

 and sagacious views of Dr Robison connected with 

 the Theory of Musical Instruments and Acoustics 

 generally, must also be passed over. 



In FELIX SAVART we find a man like Chladni who 

 was especially devoted to a single and circumscribed 

 branch of science acoustics. Hispublishedresearches 

 are almost all detached notices in the Annales de 

 Chimie, with a few memoirs in the publications of the 

 Institute ; and whilst they are very interesting, exact, 

 and instructive, I doubt whether it would be possible 

 to place the results in a connected and comprehen- 

 sive view before the reader. They are therefore rather 

 to be sought in the special articles of the Encyclo- 

 pedia devoted to the subject. In general they may 

 be stated to refer to the following topics : (1.) To 

 the manner of propagation of sound through the 

 air and through liquids, with an attempt to explore 

 the manner in which sounds spread in apartments 

 of different forms ; an enquiry as difficult as it is 

 important ; (2.) To the generation and audibility of 



musical notes. Previous authors (for example, On the prc 

 Chladni, Biot, and Wollaston) having differed mate- duct . ioi > of 



n j.i f 3'ilii f j. j musical 



nally as to the range of audibility or repeated equi- no teg. 

 distant impressions which affect the human ear as 

 musical notes, Savart used a simple method, no 

 doubt original to him, but anticipated I believe by 

 Eobison, in which a card is held near or touching 

 a revolving wheel, and the number of impulses 

 (each double) given to the air by every tooth as 

 it passes the card, is readily measured. He thus 

 found that a note occasioned by 24,000 double vi- 

 brations in a second is perfectly audible ; and, at 

 the other limit of the musical scale, from seven to 

 eight equidistant beats constitutes a sound having 

 a distinct pitch. According to Savart, two conse- 

 cutive double impulses of whatever duration are suf- 

 ficient to convey to the ear the sensation of pitch. 

 But a more elegant and accurate instrument for 

 the numeration of sonorous pulsations is the Sirene Sirene of 

 of M. CAGNIARD DE LA TOUR, unquestionably one of M - Ca e m - 

 the most exact and satisfactory additions lately made ^ ur e 

 to our experimental apparatus. In it a current of 

 air is repeatedly interrupted and renewed, giving 

 rise to a series of impulses similar to those of the 

 toothed wheel ; and this apparatus is ingeniously 

 contrived, so as to maintain its own motion, and 

 record its indications. It is by far the most accu- 

 rate known method of ascertaining the pitch of a 

 given note. It may also be worked with water, 

 llobison had also the merit of the primary idea of 

 the Sirene, by making a stopcock revolve rapidly 

 v/hilst applied to a tube emitting a blast of air. (3.) 

 Savart extended the researches of Chladni by means Savart on 

 of sand to many new cases, and with interesting *. e n vi ^ ra " 

 results ; in particular he exhibited the effects of the 80 iids. 

 unequal mechanical elasticity of crystals cut in dif- 

 ferent directions. He has also examined with great 

 care and ingenuity, the nature of the vibrations which 

 occasion the accumulations of sand on the nodal 

 lines of plates, and he comes to the conclusion that 

 they are determined by simultaneous transverse and 

 longitudinal movements (the latter of which are pa- 

 rallel to the surface of the plate). In proof of this 

 he shows that in long rods or hollow cylinders, the 

 position of the nodes is intermediate and opposed 

 upon the contrary sides of the rod or hollow cylinder. 

 Savart made many experiments on the communica- 

 tion of vibrations from one body to another ; showing 

 that the molecular movements generally preserve 

 their parallelism, so that a longitudinal vibration of 

 one body may give rise to transversal movements in 

 another ; and he applies this to the theory of musi- 

 cal instruments. 



Savart was born at Mezieres in the department of (442.) 

 Ardennes, on the 30th June 1791 ; and died some- Biographi- 

 what prematurely on the 16th March 1840. 

 had some peculiarities of temper, amongst which 

 was his unconquerable prejudice to everything Eng- 

 lish. He did not even acknowledge the intimation 



