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[Nature, Vol. xcv. pp. 645, 646, 1915.]
SOME two years ago I asked for suggestions as to the formation of an artificial hiss, and I remarked that the best I had then been able to do was by blowing through a rubber tube nipped at about half an inch from the open end with a screw elamp, but that the sound so obtained was perhaps more like an /than an s. " There is reason to think that the ear, at any rate of elderly people, tires rapidly to a maintained hiss. The pitch is of the order of 10,000 per second *." The last remark was founded upon experiments already briefly described f under the head " Pitch of Sibilants."
" Doubtless this may vary over a considerable range. In my experiments bhe method was that of nodes and loops (Phil. Mag. Vol. vil. p. 149 (1879); Scientific Papers, Vol. I. p. 406), executed with a sensitive flame and sliding reflector. A hiss given by Mr Enock, which to me seemed very high and not over audible, gave a wave-length (X) equal to 25 mm., with good agreement on repetition. A hiss which I gave was graver and less definite, corresponding bo X = 32 mm. The frequency would be of the order of 10,000 per second, more than 5 octaves above middle C."
Among the replies, publicly or privately given, with which I was favoured, was one from Prof. E. B. Titchener, of Cornell.UniversityJ, who wrote :
" Lord Rayleigh's sound more like an f than an s is due, according to Kohler's observations, to a slightly too high pitch. A Galton whistle, set for a tone of 8400 v.d., will give a pure s."
It was partly in connexion with this that I remarked later  that I doubted whether any pure tone gives the full impression of an s, having often experimented with bird-calls of about the right pitch. In my published papers I
* Nature, Vol. xci. p. 319, 1913.
f Phil. Mag. Vol. xvi. p. 235, 1908;   Scientific Papers, Vol. v. p. 486.
J Nature, Vol. xoi. p. 451, 1913.
 Nature, Vol. xoi. p. 558, 1913.
R.   VI.
22uliar. The first. ction becomes infinite when 3 = 4Ajr2, that is when  is still small. The rn occurs when 2 = 12ip, and the corresponding least negative value is o small. The first tan"1 thus passes from 0 to TT while  is still small. ie second fraction also becomes infinite when 2 = 4i/r2, there changing ;n, and again approaches zero while  is of the same order of magnitude.ened.                                                                                                                            *