Royal Institution of Great Britain, 425 



of the atmosphere, and these produce an effect according to the 

 number of impulses in a given time. If there be less than thirty 

 impulses in a second, no sound is produced ; gradually increasing 

 the number, low sounds ar« heard at first ; but they become higher 

 as the impulses are more numerous in a given time, until, when 

 above 14,000 in a second, the sounds arie so acute as to be inaudible 

 to ordinary ears. Whenever the impulses are regular and with any 

 constant velocity, then a sound is produced, having a constant and 

 determinate pitch. A very curious experiment, made by Gallileo, 

 and described in his dialogues, was quoted, which seems to have 

 been forgotten since his time. It consists in drawing the blade of 

 a penknife over a plate of copper sideways, so as to produce 

 sounds ; at the same time it is found that a series of indentations 

 are marked on the copper, which indicate the successive impulses 

 which produced the sound ; they are extremely regular, and the num- 

 ber of them will be seen, upon examination, to correspond with the 

 pitch of the sound. Mr. Wheatstone has remarked, that in all cases 

 of sound from friction, as when a bow is drawn across the strings 

 of a violin, the same series of impulses are produced, and may be 

 rendered evident, by fixing a small steel bead upon the bow ; when 

 looked at by a light or in sunshine, the bead seems to form a seriea 

 of dots during the passage of the bow. j;^ nujur-- KXior.'; !•>/ 



Hooke performed an experiment, in which a toothed wheel was 

 made to revolve, and a card or quill held against the teeth ; sounds 

 were produce^ having a regular pitch, according to the number of 

 blows or impulses given upon the card in a certain time. The 

 notes produced in this way in the lecture were very distinct, and well 

 characterised as to pitch, more especially the higher ones. 



Professor Robison produced the same effect by impulses given 

 to air alone. This was done by blowing air through a stop-cock 

 whilst the plug was rapidly revolving ; and as the motion of the 

 vilus; was more or less rapid, so the tones produced were higher or 



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An mstrument was invented some years ago by M. Cagmard de 

 la Tour, which gave very beautiful results, confirmatory of the above 

 views, and one of them was prepared and experimented with at the 

 lecture table, It consisted of a box, into the bottom of which 

 a pipe^ w^s^, 'fixed,, to btow^^^^^^^^ and in ^the top of 



which were &me(i"six*ty' K(Mes, placed in a circle, and equidis- 

 tant from each other. A metallic plate was fixed on an axis over 

 the top, so as to be made to revolve at pleasure, and was also per- 

 forated by sixty holes coincident with tho^se in the box^ Whea tba 



