358 



NATURE 



\_August i8, ]l 



I NOTICE the same phenomenon here, under the sycamore 

 trees, when they are in blossom, which your correspondent Mr. 

 Masheder observed recently under his lime trees, namely, the 

 heads and thoracic segments of severed humble bees lying on the 

 ground, with legs and wings attached, still retaining their vitality 

 in some cases, but without any trace of the abdominal segments, 

 for the sake of whose contents, no doubt, the bees \\ere 

 destroyed. We have no fly-catchers here. I suspect the tom- 

 tits, which are abundant in the vicinity of this wholesale apicide, 

 but I have no direct evidence of their guilt. R. V. D. 



Beragh, Co. Tyrone, August 15 



Migration of the Wagtail 



Apropos of recent letters on this subject in Nature, permit 

 me to note that on my voyage out to the East Indies in the 

 month of October, 187S, on board the Dutch mail steamer 

 Celebes, two wagtails alighted on the ship when not very 

 far north of the equator (the ship's course being then 

 from Aden to Padang in Sumatra). On observing them I 

 pointed them oat to a Dutch friend, who at once recognised 

 them as Kwikstails. They were rather lively, and did not appear 

 to us to be fatigued ; after staying with us for some days they 

 took their departure, but in what direction I had not the satis- 

 faction of observing. 



Without affirming positively, I believe the species was the 

 Motacilla alba. Henry Forbes 



Sumatra, June 



ITALIAN DEEP-SEA EXPLORATION IN THE 

 MEDITERRANEAN 



A FTER some delay, beyond our control, the war- 

 ■^*- steamer of the Italian Royal Navy Washington, 

 Capt. G. B. Magnanghi, R.N., left Maddalena on the 2nd 

 inst. on her thallassographic mission. Under the able 

 direction of Capt. Magnaghi, two days were devoted to 

 preliminary dredgings and trawlings in depths from 200 

 to 1000 metres, principally for testing our apparatus, 

 which works admirably. On the 4th inst. (yesterday after- 

 noon) we did our first deep-sea dredging in 3000 metres ; 

 the dredge came up empty, but I had the pleasure of secur- 

 ing, attached to the hempen tangles, a magnificent spe- 

 cimen of that strange blind Crustacean discovered by the 

 Challenger in the North Atlantic, and named U'illeniirsia 

 leptodactyla; it is no doubt one of the most characteristic 

 forms of the deep-sea fauna, and its discovery in the 

 Mediterranean is of very great importance and interest, 

 as all students of thalassography'will be fully aware, after 

 what Dr. Carpenter has written on the biological condi- 

 tions of the deeper parts of that sea. Our specimen of 

 Willcma'sia is slightly smaller than the one dredged by 

 the Challenger, and figured in Sir Wyville Thomson's 

 "Atlantic," vol. i. p. 189 ; but otherwise it differs only in 

 one or two minor details, which may be sexual differences ; 

 it was dredged off the west coast of Sardinia. 



On account of a slight mishap with our engine we have 

 anchored at Asinara for a couple of days, but shall at 

 once resume our work. Henry H. Giglioli 



Asinara, Sardinia, August 5 



KONIG'S WAVE-SIREN 

 ■pVERY musician is painfully familiar with the fact 

 ^ that two notes nearly, but not quite exactly, in unison 

 with one another, produce, when sounded together, a 

 throbbing sound commonly described as the phenomenon 

 of " beats." In the elementary theory of acoustics the 

 cause of beats is shown to be the mutual interference of 

 the two vibrations, one sound interfering with the other 

 and silencing it, when one set of waves is half a vibration 

 behind the other. Just as at certain points on the earth's 

 surface there are no tides when a high tide and a low tide 

 coming from different seas meet, so there is no sound 

 when two sets of sound-waves meet in opposite phases. 

 If the two notes differ just a little in pitch they will alter- 



nately reinforce and interfere with one another, and 

 produce the throbbing sound of beats, the number of 

 beats (or maxima of sound) per second being the same 

 as the difference in the number of vibrations per second. 

 If one tone makes m vibrations per second and the other 

 n (a slightly smaller number, being a slightly flatter tone) 

 there will be ;« — n beats per second heard. If this 

 number be not more than 3 or 4 per second the beats can 

 easily be counted. When they get as rapid as 12 or 14 

 per second they come too fast to be counted, and are very 

 harsh and grating. They are most disagreeable at about 

 33 per second ; and if yet more rapid, are heard as a harsh, 

 disagreeable, rattling sound quite different from a true 

 note. Imperfect octaves and imperfect twelfths likewise 

 cause beats ; in fact there are beats heard for any imper- 

 fectly tuned consonance in which the frequency of the 

 higher note is i, 2, 3, 4, 5, . . . or any integer number of 

 times that of the lower. 



But along with the disagreeable and throbbing pheno- 

 menon of beats there arises another phenomenon when 

 two notes not in unison with one another are simultaneously 

 sounded. This is a low booming tone, to which musi- 

 cians give the name of the "grave harmonic." If two 

 stopped organ-pipes are brought to unison, and then one 

 of them is sharpened by gradually pushing in its stopper, 

 the beats are heard first slow, then fast, then unendurably 

 rapid. But when they reach about twenty or thirty per 

 second the low booming note begins, and rises gradually 

 in pitch as the beats become too rapid to be discrimin- 

 ated. When the higher note has reached a point about 

 half-way between unison and the octave note, the beats 

 are practically imperceptible, and from this point the 

 phenomena recur again, but in inverted order, the grave 

 harmonic falls in pitch down to a low booming tone, 

 while the beats begin again to be distinguishable, grow 

 harsher, then become slower, until when the interval of 

 the octave is reached they also disappear. 



A great controversy with respect to these low tones of 

 the grave harmonics has arisen in recent years, and 

 though it smoulders from month to month, occasionally 

 blazes up into vigorous flame. The controverted question 

 is, What are these grave harmonics, and to what are they 

 due ? Also, What becomes of the beats when they occur 

 so rapidly that the ear cannot distinguish them .' The 

 answer given by Dr. Thomas Young, and by Smith in his 

 " Harmonics " (1749), was that the rapid beats actually 

 passed into the grave harmonic, just as in the generation 

 of any pure tone the separate vibrations (which, when 

 very slow, are heard as separate sounds) blend into one 

 continuous tone whose pitch depends upon their fre- 

 quency. This view is maintained at the present day with 

 great energy also by the famous acoustician Dr. Rudolph 

 Konig of Paris. On the other hand, Helmholtz has 

 emphatically maintained that the grave harmonic is not, 

 and cannot be, thus accounted for, and has given very 

 cogent reasons for thinking that it has another explana- 

 tion ; and in this view he is supported by Preyer, Lord 

 Rayleigh, Ellis, Bosanquet, and all the best English 

 physicists. Mere alternations of sound and silence, how- 

 ever rapidly they occur, cannot produce the same effect 

 on the mechanism of the ear as a pure to-and-fro motion 

 of the same periodic frequency. A tuning-fork which 

 vibrates 100 times per second will give out waves which, 

 falling on the ear, push the drumskin in, and draw it back 

 that number of times per second. But a continuous tone 

 interrupted 100 times per second by short periods of 

 silence produces quite a difterent mechanical action on 

 the mechanism of the ear. The writer of this article once 

 tried to ascertain, by the experiment of rotating a vibrating 

 tuning-fork upon its axis, whether the alternations of sound 

 and silence which are observed as it is rotated would 

 blend into a continuous tone ; but no kind of blending 

 took place. Another most conclusive proof that the beats 

 and the beat-tones are distinct phenomena is that at a 



