149 ^1 



NATURAL-PHILOSOPHICAL COLLECTIONS. 



Lecture of Mr. Faraday, before the Royal Institution of London, on Mr. 

 fVheatstone^s Discoveries relative to Linear Conductors of Sound. 



The fact of the transmission of sound through solid bodies, as in the case of a 

 metal rod placed at one extremity to the ear, and struck or scratched at the other 

 end, has been long since observed ; but as an aerial medium was formerly thought 

 to be alone capable of propagating sonorous undulations, even Lord Bacon, when 

 describing this experiment, falls into the error of attributing its transmission to 

 spirits contained within the pores of the body. The first correct observations on 

 this subject appear to have been made by Dr. Hooke, in his Micrographia, 

 (1667;) and he made an experiment through a distended wire, of sufficient 

 length to observe that the same sound was propagated far swifter through the 

 wire than through the air. Professor Wunsch of Berlin (1788) made a similar 

 experiment, substituting 1728 feet of connected wooden laths for the wire, and 

 confirmed Dr. Hooke's results. Other experiments of a similar nature were sub- 

 sequently made by Herhold and Rafn, Hassenftatz and Gay Lussac, Lamarck, 

 &c. ; but the first direct observations of the actual velocity of sound through so- 

 lid conductors were made by Biot, assisted at different times by Bouvand and 

 Martin : his experiments were made on the sides of the iron conduit pipes of 

 Paris, through the length of 951m 25 : and the mean result of two observations 

 made in different ways gave 3459 metres, or 11,090 feet per second, for the velo- 

 city of sound in cast iron. Previous to these direct experiments, Chladni had in 

 an ingenious manner ascertained the velocity of sound in different solid sub- 

 stances; and his result has been fully confirmed by calculations from other grounds. 

 His method was founded on Newton's demonstration, that sound passes through 

 a space filled with air, of a given length, in the same time that a column of air 

 of the same length, contained in a tube open at both ends, makes a single vibra- 

 tion. Chladni's discovery of the longitudinal vibrations of solid bodies, which 

 are exactly analogous to the ordinary vibrations of columns of air, enabled him to 

 apply this proposition to solid bodies, and to establish the general law, that sound 

 passes through every body in the same time in which that body, when it vibrates, 

 freely makes one longitudinal vibration. In this manner he ascertained the velo- 

 cities of tlie following substances, among others : tin 7800, silver 9300, copper 

 12,500, glass and iron 17,500, and various woods from 11,000 to 18,000 feet per 

 second. The intensity with which sound is transmitted, has been found to be 

 nearly in proportion to its velocity. Some known practical applications of this 

 principle were then noticed, such as the stethoscope, microphone, &c. In all the 

 preceding experiments, the sounds transmitted were mere noises, such as the blow 

 of a hammer, or, as in Herliold and Rafn's experiment, a single musical sound, 

 produced by striking a vibrating plate attached to the conducting wire ; and in 

 no case were any means employed for the subsequent augmentation of the trans, 

 mitted sound. Mr. C. Wheatstone was the first who tried experiments on the 

 transmission of the modulated sounds of musical instruments ; and who showed 

 that the undulations propagated through linear conductors of considerable length, 

 were capable of exciting in surfaces with which they are in connexion, a quanti- 

 ty of vibratory motion, sufficient to be powerfully audible when transmitted 

 through the air. The following observations will illustrate the nature of this re- 

 sonance or reciprocation. A sonorous body is audible in proportion to the quan- 

 tity of its vibratory surface. Thus, a plate of metal or wood is capable of pro- 

 ducing powerful sounds without accessory means ; but insulated strings, or tun- 

 ing forks, being vibrating bodies of much smaller dimensions, are scarcely audible 

 at any great distance ; but they are capable of considerable augmentation when 



