746 



NATURE 



[August ii, 192 i 



inorphous, and that, in addition, we find that KBr 

 and KCN have nearly identical molecular volumes — 

 43- 1 and 428 respectively. Thus if CN replaces Br 

 there is no appreciable change in volume, and we 

 may conclude, tentatively, that the cv'anogen radicle 

 and the bromine atom have the same size." 



Acting on the suggestion of Prof. W. L. Bragg, 

 the writer has made X-ray examinations of small 

 single crystals of KCN by the ordinary spectrometer 

 method, and of powdered crystals by the modifica- 

 tion of the method recently described by Sir W. H. 

 Bragg before the Physical Society of London. The 

 results of this preliminary investigation indicate 

 that the underlying structure of KCN is similar 

 to that of KBr, the cyanogen radicle replacing 

 the bromine atom. For instance, the strongest 

 reflection is that given by the [loo] face at a 

 glancing angle of 6° 15'. This corresponds to 

 a distance of 327 A. between the planes, and 

 the calculated mass associated with the unit cube 

 the edge of which is of this length is one-half of the 

 mass of the KCN molecule ; this is a characteristic 

 of the face-centred lattice. The first-, second-, and 

 third-order reflections from the [100] face have intensi- 

 ties which decrease in the normal way, although at a 

 greater rate than is usually the case ; the first-order 

 reflection given by the [iii] face at a glancing angle 

 of 5° 40' is relatively small, while the second-order 

 reflection at 11° 30' is normal, "as is also the first-order 

 reflection from the [no] face. These spectra corre- 

 spond to those given by NaCl, where the unit of the 

 structure consists of a cube with atoms of one kind 

 arranged at the corners and face centres, and atoms 

 of the other kind at the mid-points of the edges and 

 at the cube centre. 



The data obtained, while being sufficient to fix the 

 position of the CN radicle as a whole with respect to 

 the potassium atom, afford practically no evidence as 

 to the disposition of the carbon and nitrogen atoms 

 towards each other. So far as the lower orders of 

 spectra are concerned, the CN radicle behaves as a 

 single unit, whose power of diffracting X-rays 

 differs from that of the potassium atom. The edge 

 of the unit cube in the KCN crystal is 654 A. 

 Taking 4-15 A. as the diameter of the potassium 

 atom (Prof. W. L. Bragg, ?h\l. Mag.. August, 1920), 

 this leaves 239 A. as the width of the space, measured 

 along the cube edge, to be filled by the CN radicle. 

 The diameter of the bromine atom is 238 A. 



The results of the investigation will be published 

 in detail elsewhere. P. A. Cooper. 



Manchester University, July 28. 



An Ornithological Problem. 



Staying this last week-end with a friend at Over- 

 strand, I was much puzzled on the morning of 

 August 6 by a strange bird which I first saw sitting 

 on some low iron gates at the end of the lawn, when I 

 took it for some kind of hawk. It then settled for a time 

 on a croquet-hoop, and ultimately flew away, when its 

 long wings and tail and smooth flight again suggested 

 a hawk. My host, who had seen it before, thought 

 it might be a cuckoo, and this, when a little later we 

 saw it again in flight, seemed a probable solution. 



We did not see the bird again until the evening of 

 August 7, when during a heavy shower it appeared on 

 the lawn and perched on a croquet-hoop close to the 

 house. I then saw^ that its plumage was not grey- 

 blue, like the adult cuckoo, but a rich mottled brown, 

 and I began to think that it might be a nightjar, 

 though its beak seemed a little too long and its ap- 

 pearance in a beautifully trim garden on the edge of 

 the sea, in the daytime, out of character. Moreover, 

 both on gate and croquet-hoop it sat crosswise, not 

 lengthwise as the nightjar does on a branch. It also 

 KO. 2702, VOL. 107] 



occasionally hopped, somewhat clumsily, across the 

 lawn and regaled itself with a worm like any thrush. 



On my return to town it was suggested to me that 

 the bird might be a young cuckoo. It so happened 

 that I had never seen one, and so was not aware how 

 different the plumage is from that of the adult bird. 

 After consulting the authorities, however, such as 

 Dresser and Lilford, I am satisfied that this is the 

 right solution, for the mottled brown plumage is quite 

 in order, and the beak and the length of wings and 

 tail are clearly more those of a cuckoo than of a 

 nightjar. Moreover, we are expressly told that the 

 cuckoo when on the ground hops in an ungainly 

 fashion, whereas it is doubtful whether a nightjar 

 with its peculiarly constructed feet could hop at all. 

 The cuckoo, like the nightjar, is normally insecti- 

 vorous, but this bird might have been brought up by 

 a thrush and imitated its foster-parent's method o'f 

 dealing with worms on a lawn. Presumably the 

 young cuckoo is not ready for its long flight across 

 sea so soon as the adult bird, of whom we read, " In 

 August, go he must." George A. Macmillan. 



August 9. 



Uniform Motion in the >Ether. 



It seems to be fairly generally conceded that uni- 

 form motion relative to the aether is, in principle, un- 

 detectable by optical devices. Poincar^, for instance, 

 who did not entirely accept the positions of relativity, 

 stated as his opinion that "optical phenomena only 

 depend on the relative motions of the bodies con- 

 cerned, and this not to quantities of the order of the 

 square or cube of the aberration, hut rigorously." 



A very simple consideration, however, shows that 

 such a view is untenable. Thus, if we have a vertical 

 mirror, with a horizontal motion in its own plane 

 relative to the earth, and if a horizontal beam strikes 

 it, the angles of incidence and reflection must, as 

 measured from the moving mirror, be equal, for 

 otherwise the measured discrepancy would determine 

 the earth's motion. 



Owing to the aberration, however, these apparently 

 equal angles are not, in general, truly equal, nor are 

 they equal as measured from the earth. It is only 

 when the direction of the earth's motion is in the 

 direction of the horizontal axis of the mirror that they 

 will be equal when so measured. 



This determines the direction of the earth's motion, 

 and from the discrepancy in the other cases the 

 magnitude of the velocity could be found. 



An effect of the FitzGerald-Lorentz contraction would 

 be to distort angles, so that, for example, a measured 

 right angle, the bisector of which was in the direc- 

 tion of the earth's motion, would be greater than a 

 true right angle ; but this would not be compensatory 

 in the case of the mirrors, and would itself, in another 

 connection, serve to determine the earth's motion. 



In fact, angular measurements of the stars would 

 suffer discrepancies of a maximum of about 0001", 

 in opposite directions, at intervals of three months, 

 owing to the earth's motion in its orbit, and any 

 added motion would probably be detected if an 

 accuracy of 0001" in the measurement of large 

 angular distances could be obtained. 



As another example of a different kind, the simple 

 immersion in still or moving water of the Michelson- 

 Morley apparatus ought, theoretically, to give a posi- 

 tive result, since the water moves relatively to the 

 aether, and Fizeau's law indicates that the velocity 

 of light in moving water is not the same in all direc- 

 tions ; while if the water moves relatively to the 

 apparatus, this velocity is independent of the par- 

 ticular contractions of the latter. E. H. Synge. 



Dublin. 



