Experltmn'.i and Theory of the Harp of Eshs. 311 



much as relates to the Eoh'an harp, which, from the confined circulation of fuch works as 

 the original, muft be known to few of your readers. 



I am. Sir, 



Your obliged, 

 September 8, 1799, / P. P. 



Of the HarpofEolus. 



THIS pleafing inftrument, which has been related by fome to be a modern difcovery, was, 

 in truth, the invention of Kircher, who has treated largely of it in his Phonurgia. It is 

 an inftrument fo univerfally known, that it rhay well be prefumed unneceflary to give an 

 ^count either of its conftruiSlion or the manner of ufmg it. 



To remove all uncertainty in the order of the notes in the lyre, I took off all the ftrings 

 tut one; and, on placing the inftrument in a due pofition, was furprized to hear a great va- 

 riety of notes, and frequently fuch as were not produced by any aliquot part of the ftringj 

 often too I heard a chord of two or three notes from this fingle ftring. From, obferving thefe 

 phenomena, they appeared to me fo very complex and extraordinary, that 1 defpaired of being" 

 able to account for them on the principle of aliquot parts. However, on a more minute en- 

 quiry, they all appeared to flow from it naturally and with eafe. 



But before we proceed to examine the phenomena, let us confider what will be the effe£l 

 of a current of air rufliing againft a ftretched elaftic fibre. The particles which ftrike 

 againft the middle point of the ftring will move the whole ftring from its reftilineal pofition ; 

 and as no blaft continues exactly of the fame ftrength for any confiderable time, although it 

 be able to remove the ftring from its redtilineal pofition, yet, unlefs it be too rapid and . 

 violent, it will not be able to keep it bent; the fibre will, therefore, by its elafticity, return 

 to its former pofition, and by its acquired velocity pafs it on the other fide, and fo continue 

 to vibrate and excite pulfes in the air, which will produce the tone of the entire ftring. But 

 if the current of air be too ftrong and rapid when the ftring is bent from the redilineal po- 

 fition, it will not be able to recover it, but will continue bent and bellying like the cordage 

 of a fhip in a bri(k gale. However, though the whole ftring cannot perform its vibrations, 

 the fubordinate aliquot parts may, which will be of different lengths in different cafes, ac- 

 cording to the rapidity of the blaft. Thus when the velocity of the current of air increafes 

 fo as to prevent the vibration of the whole ftring, thofe particles which ftrike againft the 

 middle points of the halves of the ftring agitate thofe halves, as in the cafe of fympathetic 

 and fecondary tones; and as thefe halves vibrate in half the time of the whole ftring, though 

 the blaft may be too rapid to admit of the vibration of the whole,' yet it can have no more 

 effeft in preventing the motion of the halves than it would have on the whole ftring were its' 

 tenfion quadruple; for the times of vibrations in ftrings in different lengths, and agreeing;" 

 in other circumftances, are directly as the lengths; and in ftrings differing in tenfion and 

 agreeing in other circumftances, inverfely as the fquare roots of the tenfions (fee Smith 



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