NATURE 



[November 7, 1895 



the following as bearing, perhaps, the greatest resemblance to 

 the above case as regards atmospheric conditions : — 



" A la suite d'un violent orage observe pres de Wakefield, 

 le 1" mars 1774, lorsqu'il ne restait plus dans tout le ciel que 

 deux nuages peu eleves au-dessus de I'horizon, M, Nicholson 

 voyait a chaque instant des meteores semblables a des etoiles 

 filantes descendre du nuage superieur au nuage inferieur." 



October 28. George M. Minchin. 



The Dispersal of Acorns by Rooks. 



In peat-mosses, on open chalk downs, and in ploughed fields, 

 often a mile or more from the nearest mature tree, one con- 

 stantly finds acorn-husks and also seedling oaks, which last a 

 few months or, perhaps, a couple of years, and then die, the con- 

 ditions being unfavourable. It has always seemed to me, while 

 studying the origin of the existing fauna and flora of Britain, 

 that this dispersal of acorns ought to give an important clue to 

 the means by which this country was again clothed when the 

 climate became more genial after the Glacial Epoch. The oak 

 has the largest seed of any British plant, and if it can be carried 

 distances of a mile or more, it is evident that the whole of our 

 present flora may have spread more rapidly than is usually 

 imagined, and may have crossed straits and wide rivers. 



I have for several years noted the position of these seedling 

 oaks, finding them in places where no mammal would take the 

 acorns. For instance, they are common in any of the New 

 Forest peat-bogs that are within a mile of an oak-tree. They 

 are common also in some places on the top of the escarpment of 

 the South Downs, half a mile from oaks, and 300 or 400 feet 

 above them. They are always associated with empty acorn- 

 husks, stabbed and torn in a peculiar way. 



In October and November rooks feed in the oak-trees, and I 

 have long felt convinced that they were mainly responsible for 

 the dispersal of acorns, though it is not easy to catch them 

 actually doing it. On October 29 of this year I was successful. 

 In the middle of an extensive field, bordered by an oak-copse 

 and scattered trees, a flock of rooks was feeding and passing 

 singly backwards and forwards to the oaks. On driving the 

 birds away, and walking to the middle of the field, I found 

 hundreds of empty acorn-husks, and a number of half-eaten 

 pecked acorns, which had not had time to change their colour — 

 a cut acorn changes colour on exposure to the air like a cut 

 apple, though not quite so fast. This showed that the birds had 

 been disturbed in the middle of their feast, for the marks on the 

 acorns were quite unlike those made by a rodent or any 

 mammal. They were stabbed and pecked, and the husks were 

 torn off in strips, usually starting from a puncture. It was also 

 noticeable that many of them were not shed acorns, but were 

 accompanied by acorn-cups, the stalks of which had been bitten 

 to tear them off the trees. This was singular, for the ground 

 beneath the trees was covered with shed acorns. The rooks, 

 however, were in the trees, not under them, and the reason for 

 the selection of acorns in cwps is probably that they are easier 

 to carry — a shed acorn must be an awkwardly large and slippery 

 thing for a rook's beak, one with a stalk will be more con- 

 venient. Several uninjured acorns were found, and most of the 

 remains occurred on smooth spots of short turf — places where a 

 slippery acorn might conveniently be pecked without being lost. 

 One almost uninjured acorn had been driven by a single peck 

 deep into the soft soil of a mole-hill. 



It might be thought that it would be much simpler for the 

 rooks to feed on the ground beneath the trees. Some of them 

 apparently do so ; but the majority seem always to carry the 

 acorns into the open. The rook is a suspicious bird, quarrel- 

 some, and a born thief. He seems particularly to object to a 

 comrade watching him from any post of vantage, and the rooks 

 when among the oaks, for some reason or other are always 

 quarrelling, notwithstanding the abundance of food. An acorn 

 dropped on rough ground or in a peat-moss would stand a great 

 chance of being lost in some crevice or soft place ; but the oak 

 seeds so freely, that the bird need not waste time trying to 

 recover the lost acorn — there are plenty more on the tree. 



In this way oak-woods must spread rapidly. But we still 

 want observations as to the distance to which acorns can be 

 carried. I have seen seedling oaks at a distance of a mile from 

 the nearest mature tree (not necessarily the tree from which the 

 acorn came), and have found the characteristically torn husks 

 somewhat further away. Do rooks roosting in elm-trees ever 

 carry home acorns for supper ? There used to be a number of 



NO. 1358, VOL. 53] 



rooks which roosted in elms near Brighton in the autumn and 

 winter, but crossed the Downs to feed in the Weald. I have 

 often watched them returning at dusk. Do they ever bring, 

 acorns from that distance ? This flock may have been re- 

 sponsible for the seedling oaks near the edge of the Downs ; and 

 if it could occasionally bring an acorn still further, to Brighton, 

 it is evident that the oak may have crossed the Strait of Dover, 

 when it was somewhat narrower, and that Britain, as far as the 

 oak shows, may have been continuously an island since the 

 Glacial Epoch. Clement Reid. 



On the Audibility of Fog Signals at Sea. 



In Nature of August 8, attention was called to some 

 recent investigations, published in Hansa, on the inaudibility of 

 fog-horns at sea within certain zones surrounding the signal, 

 although the horn is distinctly heard outside of such regions. It 

 seems strange to me that I can find nowhere suggested that this 

 may be a phenomenon of interference similar to that suggested 

 in light by Dr. Lloyd of Dublin {Trans. Roy. Msh Acad., vol. 

 xvii.). 



If we let X equal the distance of the observer from the signal, 

 h and y the heights of the signal and the observer, respectively, 

 above the level of the sea, and S the linear difference between 

 the paths of the reflected and the direct rays of sound, then 



X = JL j 8*-482 (;^2 + ^2) + 16 /zV ! ^ = ^-^ 

 20 I ) 5 



approximately. An attempt to apply this formula to the observ- 

 ations recorded in the Report of the American Light-House 

 Board, published in 1894, was foiled by the lack of sufficient 

 data for substitution in the above formula. However, if 

 h = 100 it., y = 30 ft., and 5 be taken for a wave-length of 

 2 ft. (which are probable values for the variables), then we 

 would expect minima of sound at I'l and 1/3 miles, the 

 maximum between these being at a distance of half a mile from 

 the source of sound, which quantities are of the right order of 

 magnitude. These distances might be modified considerably by 

 refraction, the wind, and to some extent by the tide. When 

 there are two minima, as in the Hansa experiments, this seems 

 a much more probable explanation than that by refraction alone 

 generally offered, and it explains the phenomenal loudness out- 

 side the silent area. The above equation shows that the 

 boundaries of the silent regions in vertical planes are hyperbolas, 

 which is essentially different from what the refraction theory 

 gives. F. E. FowLE. 



Washington D.C., U.S.A., October 21. 



To Friends and Fellow Workers in Quaternions. 



In Nature for October 3, 1895, ^^here is a letter, signed by 

 P. Molenbroek and Shunkichi Kimura, on promoting the study 

 of quaternions and allied systems of mathematics. I notice that 

 this has not, as yet, been responded to in Nature. I do not 

 think that the subject should-be allowed to drop ; but that some 

 permanent good should be done to science by making this branch 

 of mathematics part of the compulsory course of study for 

 students for the highest honours in mathematics in our univer- 

 sities and uuniversity colleges, in the hope that more workers 

 may follow the subject up afterwards. 



Unquestionably the calculus is of very great value in the 

 higher natural philosophy, and in every sense will repay the 

 trouble bestowed upon it, though I speak in all meekness and 

 not as an eminent authority on the subject. May we hope for 

 some information as to what form the literature of the Inter- 

 national Association will take ? G. H. J. Hurst. 



Eton College, Windsor. 



The Colours of Mother-o'-Pearl. 

 Had Mr. C. E. Benham given his address when writing to 

 you on this subject (Nature, vol. Hi. p. 619), I should 

 merely have taken the liberty of sending to him direct a 

 copy of a paper entitled, "Prof. Blake and Shell-growth in 

 Cephalopoda" {Ami. and Mag. Nat. Hist., ser. 6, vol. i. 

 pp. 421-427, June 1888), in which similar arguments to those 

 of Mr. Benham were adduced. Now, however, perhaps you 

 will permit me to refer Mr. Benham to Dr. W. B. Carpenter's 

 report on shell-structure {Brit. Assoc. Rep., 1844, p. 11). As for 

 the text-book writers, who usually support their explanation of 

 the iridescence of mother-o' -pearl by reference to the theory of 



