May i, 1913] 



NATURE 



21 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Atmospheric Electrification during Dust-storms. 



Observations which have been made at the Patna 

 College since the beginning of the present year indi- 

 cate that the remarkably high negative potential 

 gradient noticed as a feature of South African dust- 

 storms in Prof. Rudge's letter, published in Nature 

 of March 13, also exists while the ordinary westerly 

 winds of the hot weather are blowing in north India. 

 At Patna they usually blow from about 9 a.m. to 

 6 p.m. from the middle of March until June, and they 

 raise a great deal of dust, though the real dust- 

 storms seldom occur so far to the east of India. This 

 year they are unusually late in starting, and their 

 place has been taken on most days by east winds, 

 during which the potential gradient is of the ordinary 

 positive type and magnitude. 



So far, measurements have only been made with 

 a portable electrometer and water dropper, the latter 

 mounted on a post 56 metres from the ground with 

 good exposure. Until March 15 nothing abnormal 

 was noticed. The positive gradient was rather lower 

 than usually recorded, seldom exceeding 60 volts per 

 metre. Negative gradients of about 30 were 

 measured on the afternoons of January 19 and March 

 11, but only lasted for a few minutes. At 3 p.m. 

 •on March 15, with a hot west wind blowing in gusts, 

 the negative gradient was about 120 volts per metre. 

 Under similar conditions on March 16 it rose to 

 above 400 during some of the stronger gusts. On 

 March 23, when the wind had once more shifted to 

 the west, the negative gradient was too high to be 

 measured with the electrometer. 



Since the copy of Nature containing Prof. Rudge's 

 letter reached me, April 7 has been the only day with 

 a strong west wind. On the morning of April 6 

 a" squall, due to a local disturbance, blew from the 

 north from 9 to 11 a.m. It raised a certain 

 amount of dust on the sandbanks of the Ganges, 

 but the atmosphere was unusually clear. While the 

 wind was blowing hard the negative gradient was 

 about 180 volts per metre, but this gradually 

 diminished as the wind died away, and the usual 

 positive gradient was re-established about 11.30 a.m. 



A summary of the observations made on April 7 

 is given below : — 



Time Gradient (volts per metre) 



7.15a.m.: ... ..- ... 24 + 



9.IO j2- 



11.30 ... ... ... ... 420- rising to 500- 



12 noon ... ... much above 500- 



1 p.m ,, ,, 500- 



2 ... ... ... ... ... about 15CO- 



3 , 1650- 



5.30 1S0- 



6.30 3° + 



8.30 above 250 + 



10 .. ... ... .. ... about 250 + 



10.20 I3° + 



12 100 + 



12.30 ... 36 + 



The rough value at 2 p.m. was obtained by a spark 

 micrometer, the sparks being from 19 to 2 mm. in 

 length. At 3 p.m. the water dropper was mounted 

 on another post i-6 metres from the ground, and it 

 was just possible to measure the potential with the 



NO. 2270, VOL. 91] 



electrometer. After 10 p.m. the wind began to veer 

 to the east, and it has remained in that quarter. 



It is evident that a continuous record of these 

 changes would have resembled Prof. Rudge's record 

 of a severe dust-storm, except that the latter does not 

 show such marked signs of a high positive gradient 

 after sunset. Later on in the season even higher 

 negative gradients will probably be observed, for the 

 conditions on April 7 were scarcely typical of the hot 

 weather, the temperature not rising above 94 and 

 the clouds of dust not dense. 



More measurements are, of course, necessary, but 

 at present it does not appear unreasonable to suppose- 

 that from about 9 a.m. to 6 p.m. on the majority of 

 days from March or April until June the potential 

 gradient over a large portion of north India is re- 

 versed, and that under these conditions the negative 

 gradient is from ten to fifty times as great as the 

 ordinary positive change. 



V. H. Jackson. 



Bankipore, April 9. 



X-Rays and Crystals. 



In my former letter of March 18 (published 

 in Nature of April 10) I briefly pointed out 

 that the transmitted beams of X-rays may 

 be made visible by means of an ordinary fluorescent 

 screen. The results of further experiments by visual 

 method are favourable for the explanation suggested 

 by Barkla and Bragg, in so far as the planes rich 

 iii molecules or atoms behave as reflecting planes 

 for rays at grazing incidence. 



A piece of colourless transparent fluorspar, crystal- 

 lised in regular octahedron, and rock-salt in the form 

 of a cube, were examined, with an incident beam of 

 1 cm. diameter. As already noticed, groups of trans- 

 mitted beams are arranged on circular cones, always 

 in contact with the incident beam, having their 

 common vertex in the crystal, and their axes fixed 

 relative to it, so that all the spots belonging to a 

 certain cone converge into the central incident spot, 

 as the axis corresponding to the cone approaches the 

 incident beam. Moreover, the elongated spots are all 

 directed towards the point of the cone diametrically 

 opposite to the incident spot. By rotating the crystal 

 about one of its principal axes, or about an axis 

 bisecting the angle between two principal axes, the 

 position of the axes of these cones was determined, 

 leading to the result that all these axes correspond 

 with the lines of intersection of several planes "rich in" 

 reflecting particles, if we assume that these points are 

 arranged in a simple space-lattice. The number of 

 spots belonging to every cone may also be accounted 

 for on this assumption. Even the brightness seems 

 to conform with the "richness" of these points in 

 the corresponding plane. 



I was also able to reconstruct graphically the com- 

 plete sets of spots shown in the -photographs obtained 

 by Laue, Friedrichs, and Knipping (Figs. 5 and 7) 

 on the above assumption. Details of the investiga- 

 tion will appear in the near future in the Proceedings 

 of the Tokyo Mathematico-Phvsical Society. 



T. Terada. 



Physical Institute, Tokyo, April 6. 



The Use of Alcyonarians as Money. 



There has just been presented to the Royal Scottish 

 Museum by Dr. E. MacKenzie, of Espiritu Santo, 

 New Hebrides, a large Coelenterate colony found on 

 the shores of the island after a storm. Dr. MacKenzie 

 supplies the information that such colonies are held in 

 great esteem by the natives, who use them as charms. 



