214 M. Terquem mi the Vibroscope Tonometer. 



The only practical difficulty which I have encountered (and 

 this I think I am on the point of surmounting) is in the mode 

 of fixing the cursors on the diapasons. In fact they should be 

 fixed there in a quite invariable manner, so that the sound may 

 not suffer any variation of pitch when the diapason is set in 

 vibration; this does not always happen, though the differences 

 observed are very slight. In the second place, the cursors must 

 be restored to the same position with mathematical accuracy. 



I have already verified a certain number of the divisions traced 

 by M. Konig on my diapasons from his tonometer, and have 

 found only differences amounting to not more than a few hun- 

 dredths of a vibration per second, which shows the degree of 

 confidence that may be placed in the determinations made from 

 the tonometer constructed by him. 



By this method, then, it will be possible, with more facility 

 than by the old process, though not without sustained attention 

 and numerous determinations, to divide the interval from ut 2 

 to ut 3 into sounds differing one from another by two vibrations, 

 or even by one only, if it be desired. At the least, by the em- 

 ployment of auxiliary diapasons with cursors, the same proce- 

 dure will permit the verification of the accuracy of the divisions 

 of the standard diapasons mounted in the form of a vibroscope, 

 and to construct a table of corrections if the divisions are not 

 perfectly exact. 



The vibroscope tonometer once constructed and verified, in 

 order to determine the pitch of any sound whatever, it will be 

 sufficient to place one of the diapasons of which it consists by 

 the side of the vibrating body in such a way that the vibrations 

 of the one shall be perpendicular to those of the other. On this 

 body (which may be a rod, a plate, a string, &c.) some particles 

 of antimony powder are to be fixed; then the cursors will be 

 shifted until an acoustic curve of well-recognizable form is ob- 

 tained, such as that due to vibrations in the ratio of 1 : 2, 1:3, 

 1 : 4, 1 : 5, . . . . If the sound studied is higher or deeper than 

 those comprised in the interval from ut 2 to ut 3 , it is not even 

 necessary to obtain the absolute fixity of the curve ; in fact, by 

 determining the duration of the period of return of the same 

 figure, we can know the difference between the number of vibra- 

 tions of the sound and that which it ought to have in order that 

 that fixity may exist. 



With this tonometer I purpose to resume, with more accuracy 

 than has hitherto been attainable, the study of the vibrations of 

 rigid bodies, particularly plates, in order to arrive, if possible, 

 at elucidating the question, still so much controverted, of the 

 propagation of sound in bodies which present at least two di- 

 mensions of the same order of magnitude. 



