INSTRUMENTS FOE EXPERIMENTS ON SOUND. 143 



instruments, he placed a paper and a tracing-point, with 

 blackened tissue behind, so that when the string vibrated the 

 point pressed against the paper and produced curves. You 

 thus can not only measure the string and get the ratios, but 

 you secure permanent vibrations by means of the rotating 

 bow, and you can also print them off on a strip of paper which 

 travels slowly in front by means of the hand, or as here, 

 by means of a weight, so as to bring it gradually past the 

 vibrator. 



There are other modes of exciting strings besides strik- 

 ing them, such as by bowing ; of course many instruments 

 act in this way. For observations of an acoustical cha- 

 racter bowing is not so good ; it is apt to produce partial 

 vibrations. We may also excite strings by the impact of 

 the air. There are specimens of struck strings in the piano- 

 forte actions which are exhibited. Bowing you are all 

 probably familiar with. The impact of air, if not entirely a 

 new discovery, has only lately been put to practical use. I do 

 not propose to go into it to-day, because my friend Mr. Baillie 

 Hamilton will deliver a separate lecture upon what he terms 

 "seolian" modes of producing sound, in which the combi- 

 nation of a string with a reed brings out new and beautiful 

 characters of tone. Strings when struck produce many upper 

 partial tones, according to the place where they are struck, 

 according to the nature of the stroke, and according to 

 the density, rigidity, and elasticity of the string. I must refer 

 you to Helmholtz's great work for further details on that point ; 

 only noticing what pianoforte-makers have discovered by 

 experience, and what Helmholtz has explained theoretically, 

 that if the hammer strike the string in the pianoforte at 

 about one-eighth or one-ninth from one end certain dissonant 

 upper partial tones are excluded and a much finer effect is 

 secured. The second form of bowing the string, as illus- 

 trated in violins and other instruments, was examined by 

 Helmholtz by means of what he terms the vibration mi- 

 croscope, an ingenious plan for producing to the eye of 

 a single observer exactly what I have shown you on the 

 screen. He sets a string into vibration, fixing a small grain 

 of some white substance, generally starch, on it, and looks 

 at it through a microscope which, instead of having a 

 fixed object-glass, has the object-glass mounted on the prong 

 of a tuning-fork. That tuning-fork is made to vibrate in 



