4IO 



NATURE 



[December 12, 1912 



The while colour of the berg is due to innumerable 

 air bubbles in the ice, and not to snow on the surface. 

 An iceberg is very deceptive in this way. While it 

 looks quite soft, the ice is so hard as to make it diffi- 

 cult to chop with an axe. The ice water which 1 

 prepared for drinking on board ship with iceberg ice 

 effervesced like soda-water, merely due to the libera- 

 tion of the air from the melting ice. It is possible 

 that the sudden disappearance of bergs with a loud 

 report is due to their explosion from accumulated air 

 in the interior. One berg which 1 studied was cast- 

 ing off small pieces, apparently by the pressure of 

 the pent-up air. 



Effect of Land. 



While icebergs send the temperature of the sea up, 

 the coast-line sends it dow'n. 1 believe this to be due 

 to the action of the land in turning up the colder 

 under-water. My observations show this effect not 

 only here, but on the English and Irish coasts. 



From the point of view of the safety of our St. 

 Lawrence route, the effect of land is most important. 

 The iceberg causes us very little worry because we 

 have only a very short ice track, but to find means 

 whereby the presence of land can be determined is of 

 the greatest importance. A full account of my experi- 

 ments is being published by the Canadian Department 

 of Marine. H. T. B.\rnes. 



McGil! L'niversity, November i6. 



The Bending of Long Electric Waves Round the Globe. 



J HA\E just noticed (very belatedly) that in your 

 reprint of Dr. Fleming's admirable opening of the 

 British Association discussion of the problems of wire- 

 less telegraphy, there occurs a passage that raises an 

 objection to a certain mathematical result of mine. 

 Dr. Fleming's opinion in all matters radio-telegraphic 

 is of such great weight that his objection, whether 

 sound or not, is sure to prejudice the fair considera- 

 tion of a hypothesis I have based on the mathematical 

 result in question, and since the objection has obtained 

 the wide publicit}' of your columns while my own 

 account of the matter has not, I trust you will allow 

 me space to comment upon it. Comment seems 

 especially necessary on account of Dr. Fleming's 

 eloquent advocacy of certain rival hypotheses. 



Put briefly, the theorem is to the effect that the 

 velocity of long electric waves through air containing 

 charged ions is greater than the velocity through un- 

 ionised air, and this leads to a hypothesis for explain- 

 ing, among other things, the propagation of electric 

 waves over the convexity of the globe. In forming 

 the electromagnetic equations I took the average 

 dielectric constant of the ionised air to be the same 

 as that of the un-ionised air, following in this respect 

 the example of previous writers on similar problems. 

 It is to this customary assumption that Dr. Flem- 

 ing's objection applies. 



In rebutting the objection there are several plain 

 courses. For example, I might recall that the formula 

 I deduced for the increase of velocity mav also be 

 obtained from the accepted theory of " anomalous " 

 dispersion — a theory in which the influence of a finite 

 change of the dielectric constant is considered to be 

 negligible. But in the present instance it seems pre- 

 ferable to take another course, and to ask, plainly. 

 Why should the presence of electrified molecules in 

 the number required by my hypothesis affect the di- 

 electric coefiicient used in the differential equations? 

 It must be noticed that the concentration of the ions 

 demaiided for bending a ray to fit the curve of the 

 earth is of the order lo' ions per c.c, assuming the 

 ion>_ to be molecular in size ; and thus the proportion 

 of ions to molecules is of the order lo-". It 

 appears to me most unlikely that such a small propor- 

 NO. 2250, VOL. 90] 



tion of ions can affect the real dielectric coetficient of 

 the medium, especially in view of the fact that there 

 does not seem to be any direct or indirect evidence 

 based on experimental or theoretical knowledge of 

 gases that i-an be held to support such a view. 



I may add that I am quite well aware of many- 

 real difficulties confronting the hypothesis. I am not 

 now writing in reference to any of those, but wish 

 merely to point out that the objection urged by Dr. 

 F'ieming is, so far as I can see, a remotely conjectural 

 one. W. H. Eccles. 



University College, Gow-er Street, W.C., 

 December 2. 



The Specular Reflection of X-rays. 



It has been shown by Herr Laue and his colleagues 

 that the dittraclion patterns which they obtain with 

 X-rays and crystals are naturally explained by assum- 

 ing the existence of very short electromagnetic waves 

 in the radiations from an X-ray bulb, the wave length 

 of which is of the order 10- ° cm. The spots of the 

 pattern represent interference maxima of waves dif- 

 fracted by the regularly arranged atoms of the crystal. 

 Now, if this is so, these waves ought to be regularly 

 reflected by a surface which has a sufficiently good 

 polish, the irregularities being small compared w-ith 

 the length 10- " cm. Such surfaces are provided by 

 the cleavage planes of a crystal, which represent an 

 arrangement of the atoms of the crystal in pa_rallel 

 planes, and the amount by which the centres of atoms 

 are displaced from tlieir proper planes is presumably 

 small compared with atomic dimensions. 



In accordance with this, the spots in Laue's crys- 

 tallographs can be shown to be due to partial reflection 

 of the incident beam in sets of parallel planes in the 

 crystal on which the atom centres may be arranged, 

 the simplest of which are the actual cleavage planes 

 of the cr^'stal. This is merely another way of look- 

 ing at the diffraction. This being so, it was sug- 

 gested to me by Mr. C. T. R. Wilson that crystals 

 with very distinct cleavage planes, such as mica, 

 might possibly show strong specular reflection of the 

 rays. On trying the experiment it w'as found that 

 this was so. A narrow pencil of X-rays, obtained by 

 means of a series of stops, was allowed to fall at an 

 angle of incidence of So° on a slip of mica about one 

 millimetre thick mounted on thin aluminium. A 

 photographic plate set behind the mica slip showed, 

 when developed, a well-marked reflected spot, as well 

 as one formed by the incident rays traversing the mica 

 and aluminium. 



Variation of the angle of incidence and of the 

 distance of plate from mica left no doubt that the laws 

 of reflection were obeyed. Only a few minutes' expo- 

 sure to a small X-ray bulb sufficed to show the effect, 

 whereas Friedrich and Knipping found it necessary 

 to give an exposure of many hours to the plate, using 

 a large water-cooled bulb, in order to obtain the trans- 

 mitted interference pattern. By bending the mica into 

 an arc, the reflected rays can be brought to a line 

 focus. 



In all cases the photographic plate was shielded by 

 a double envelope of black paper, and in one case with 

 aluminium one millimetre thick. This last cut off the 

 reflected rays considerablv. Slips of mica one-tenth of 

 a millimetre thick give as strong a reflection as an 

 infinite thickness, yet the effect is almost certainly 

 not a surface one. Experiments are being made to 

 find the critical thickness of mica at which the re- 

 flecting power begins to diminish as thinner plates 

 are used. The reflection is much stronger as glancing 

 incidence is approached. W. L. Bragg. 



The Cavendish Laboratory, Cambridge, 

 December 8. 



