November i8, 1909] 



iVA TURE 



69 



carbon in series four of tlie periodic table, namely, 

 titanium, zirconium, cerium, and thorium. Indeed, 

 '■ carbonaceous matter " is itself recorded by Prof. Rosen- 

 busch (•• Rock-making Minerals," p. 197) as giving rise 

 lo pleochmic halos in andalusite. Of course, this might 

 imply nothing more than that chemical similarity leads to 

 similar impurities {e.g. thorium), to which the radio- 

 activitv may be due. It is not easy to see how such an 

 e.\planation would tit the case of carbon, but otherwise it 

 would readily account for the fact that halos are not 

 always seen round the minerals mentioned above, and that 

 thev may occur round some crystals and yet not round 

 others in the same rock. They are most common, indeed 

 practically constant, round zircon and orthite, but are con- 

 liiud to a lew occurrences of the abundant minerals sphene, 

 .apatite, and epidote. 



.\nother point which seems to deserve emphasis is the 

 fact that, from a geological point of view, the radio- 

 activity of thorium must surely be a far more potent factor 

 than that of uranium and its derivatives. For there is no 

 mineral which occurs in ordinary rocks which appears to 

 contain uranium in quantities appreciable by chemical 

 methods, whereas, as will be inferred from what has been 

 already said, thorium is of extremely wide distribution. 



F. p. Mennell. 



Bulawayo, Rhodesia, October iS. 



Magnetic Storms. 



In his letter (Nature, November 11) Dr. Simpson raises 

 an argument as to the absence of corresponding changes 

 in the electrical potential gradient during magnetic storms 

 which, if admitted, would, I imagine, prove a serious 

 (lilTiculty in the " electron stream " theory of magnetic 

 disturbances. On the assumption that the arriving stream 

 induces an opposite charge which resides at the earth's 

 surface, we can agree with Dr. Simpson's calculation, 

 except that, since there are two current sheets of opposite 

 sign, the potential gradient ought to be half what he 

 iinds. 



It seems to me most unlikely that the induced charge 

 resides at the earth's surface. The atmosphere is slightly 

 conducting, and it is throughout it that the induced charge 

 distributes itself. It is well known that a very slightly 

 conducting shell will with great rapidity act as a perfect 

 electrostatic screen, but, on the contrary, fails to screen 

 magnetic effects. In his paper (Phil. Trans., A, 1908) on 

 terrestrial magnetism, Dr. Schuster points out that the 

 normal conductivity of air at the earth's surface is about 

 10--', while at a height such that the pressure is i dyne 

 per sq. cm. the conductivity would be about 10- ". For 

 , such a conductivity the time constant of decay would be 



y about 10-' of a second, or, in other words, practically 

 complete electrostatic screening would be established in 

 about one-thousandth part of a second, and correspond- 

 ingly the slight initial magnetic screening would then 

 cease. We have thus a simple explanation of the absence 

 of direct electrostatic effects at the surface of the earth 

 due to " electron streams " several hundred kilometres 

 above the earth. The earth currents which do accompany 

 magnetic storms are thus referred, not to electrostatic 

 induction, but to change of magnetic induction at the 

 earth's surface. George W. W.-slker. 



Eskdalemuir Magnetic Observatory, November 12. 



The Photometric Measurement of the Obliquity Factor 

 of Diffraction. 



In vol. Ixxviii. of Nature (May 21, 1908. p. 55) was 

 published a note on " Secondary Waves of Light," in 

 which I described the diffraction effects produced by an 

 obliquely held rectangular aperture or reflecting surface, 

 and pointed out that the observed distribution of illumina- 

 tion in the pattern was not in accordance with that deduced 

 in the ordinary way. I indicated an explanation of the 

 discrepancy, that it was due to the variation of the 

 obliquity factor of diffraction within the limits of the 

 pattern. 



The interest of the observations lay in the fact that 



such an effect had never been noticed before, and that the 



observations enabled us actually to trace the variation of 



the amplitude of vibration from point to point on 



NO. 2090, VOL. 82] 



Huygens's secondary waves. A full description of tlie 

 effect and a mathematical investigation were published in 

 the Philosophical Magazine for January. 



The effect observed was that the intensities of illumina- 

 tion in corresponding bands on opposite sides of the central 

 band in the unsymmetrical pattern were unequal. A 

 photometric investigation of this difference in illumination 

 has been carried out. The method was to use revolving 

 sectors to reduce the illumination in one of the two bands 

 to be compared, so as to make them both of equal bright- 

 ness. The following table illustrates the comparisons 

 made : — 



j,j Qf Ratio of illumlna- Ratio of ilUimina. Ratio of illumira- 

 ' tion according to tion actually lion calculat> d 



" ■ ordinary theory determined from obliquity 



1 ... 1-00 ... 1-66 ... r6i 



2 ... i-oo ... i-8i ... 198 



3 ... I 00 ... 2'65 ... 243 



4 ■■. 100 ■■■ 3'25 ••■ 3'27 



The obliquity law demonstrated by these measurements 

 is that, in the hemispherical wavelets emitted by each 

 element of a transmitting aperture or reflecting surface 

 upon which waves are incident at any angle, the ampli- 

 tude of the light vector is, at any point in the plane of 

 incidence, proportional to the cosine of the angle made by 

 the line joining that point and the element, with the normal 

 to the plane of the element. C. V. Ramax. 



Post-Box 59, Rangoon. 



Mendelian Heredity : A Correction. 



I SHOULD be glad of an opportunity of correcting the 

 following errors in my book " Mendel's Principles of 

 Heredity " : — 



On p. 35, Davenport's result regarding rumplessness in 

 fowls is accidentally inverted. The character, according 

 to him, is recessive, not dominant as stated by me. I 

 have to thank Prof. Arnold Lang for this correction. 



The other, and more serious, error is in the description 

 of Fig. 34 of the second impression of the book (p. 231). 

 In giving a' tentative scheme for the descent of colour- 

 blindness, I there stated that a male homozygous for 

 colour-blindness could be produced by the mating of two 

 colour-blind parents ; but if the scheme is right, it evidently 

 follows that such a male cannot be formed even from 

 that mating. W. Bateson. 



November 12. 



The Functions of the Martian Canals. 



With reference to the recent paper by Dr. Pocklington 

 before the Royal Society, on the functions of the Martian 

 canals, a notice of which appeared in Nature of 

 November 11 (p. 58), I should like to suggest that these 

 canals may perhaps be used for power-storage purposes. 

 In Mars, possibly, there are seasons of winds or monsoons 

 during which the upper reaches of the canals would be 

 pumped full by innumerable windmills, and the power thus 

 stored utilised during calm seasons, and transmitted elec- 

 trically for lighting, heating, and general power purposes. 

 For a population which had exhausted all its mineral fuel, 

 which possessed no extensive ocean, and whose soil and 

 climate were unsuitable for the growth of fuel, this would 

 indeed appear to be the only means of obtaining heat and 

 power. The same canals could serve the triple purposes of 

 communication, power, and irrigation. H. F. Hunt. 



7 Officers' Row, Pembroke Dock, Wales, 

 November 13. 



GR.iVITY SURVEY.' 



THE two publications described below afford a re- 

 markable example of the value of an Inter- 

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 in the rijjht way. The sumptuous institute upon the 

 Telegraphenberg at Potsdam is the home of the 



1 "Survev of India ; Professional Paper. No. 10. The Pendulum Opera- 

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 an appendix by A. Strahan, F.R.S. Pp. ix4-ig6. (Dehra Dun, rgoS.) 



"Deutsche Piidpolar Expedition, 1901-^." Band i., Geographic ; Heft 

 iii., Die Schwerkraftsbestimmungen der Deutschen Siidpolar Expetiitjon. 

 By E. von Drygalski "und L. Haasemann. Pp. 285-363. (Berlin: Georg 

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