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216   Life and Letters of Francis Galton

older people, and cites a Dorsetshire proverb "that no agricultural labourer who is more than forty'years old, can hear a bat squeak." He distinguishes between the sharpness of hearing and the hearing of high notes, and indicates that the position of the whistle-opposite to the auricular orificemay be of importance.

Dalby `the aurist' had already used one of Galton's whistles for diagnosis, and Galton himself had tried experiments with them on all kinds of animals at the Zoological Gardens and on insects. He put one of his whistles at the end of a hollow walking-stick which had a bit of india-rubber piping under the handle, brought the stick as near as was safe to the animal's ear, and when it was accustomed to it, squeezed the tube, and observed whether it pricked its ears. If it did, it probably heard the whistle. Cattle and ponies, much more than horses, hear high notes. If you pass through the streets of a town, working the walking-stick whistle, all the little dogs turn round, but it does not seem to have any effect on the large ones.

"Of all creatures I have found none superior to cats in the power of hearing shrill sounds. It is perfectly remarkable what a faculty they have in this way   You can make a cat, who is at a very considerable distance, turn its ear round by sounding a note that is too

shrill to be audible by any human ear."

Galton attributes this faculty, in cats to natural selection, differentiating them so that they can hear the shrill notes of mice and other animals they need to catch. Some of Galton's audience at the conference heard the high notes of his whistles, others failed to catch them at all. Among the former was Alexander J. Ellis, translator of Helmholtz's Lehre von der Tonempfindunge-n, who stated that he heard all the high notes perfectly.

It is clear that very useful work might be done to-day by testing the members of families and forming pedigrees for cases in which there is a faculty for hearing very high notes, and probably Galton's whistles would be an adequate means of investigation. I do not remember ever seeing a frequency curve for a large general population of the limit of audibility'.

An addendum to the above paper on whistles was contributed to Nature by Galton in March, 1883. He notes that while his little whistle, set at •14 of an inch, would give about 24,000 vibrations per second if air were puffed through it, the vibrations will be some 86,500 a second if hydrogen be used, because the number of vibrations per second is inversely proportional to the square root of the specific gravity o_f the gas blown through, and hydrogen is thirteen times lighter than air. Galton tested first with coalgas, the specific gravity of which is not much more than half that of common air. He found that a length of • 13 of the whistle gave him personally no audible note for air; but he heard the note at -14; he could for coal-gas get no audible note at •24. Galton suggests that the whistle-lengths at limit of audibility, being as •14 to •25, or as •56 to 1, are nearly in the ratio of •60 to 1, or the specific gravities. But if the audibility depends on the period and not the square of the period, •56 to 1 should be as the square roots

1 Galton's published data do not really provide material for such a curve (see our p. 221). 2 Vol. xxvii, p. 491, corrected Vol. xxvin, p. 54.