196 



NATURE 



[May 8, 1919 



Englishmen. The latest model of the Coolidge tube 

 acts efficiently as its own rectifier, and the high- 

 tension outfit is correspondingly simplified. 



In simple justice one must add that the scientific 

 ■energies of this country have been far more distracted 

 and dislocated by the war than have those of America. 

 But the Department of Scientific and Industrial 

 Research ought to have no hesitation in adding to 

 the list of investigations it has tackled during and 

 since the war. 



To return to the question of future developments of 

 the X-ray scrutiny of metals. The question of the 

 light alloys is relatively simple, but for the heavier 

 metals we need photographic plates and fluorescent 

 screens of much higher efficiency. In addition, we 

 need a method of generating X-rays in far greater 

 abundance and of far greater hardness (i.e. shorter 

 wave-length) than we have at present. In this con- 

 nection every investigator and user ought to develop 

 the habit of precise measurement of both current 

 through the tube and, more important, the potential 

 difference across the terminals. The voltage is com- 

 monly left to be inferred as well as may be from the 

 alternative spark-gap between electrodes the size and 

 shape of which are rarely mentioned. Or, more fre- 

 quently, the hardness of the rays is given in some 

 arbitrary unit difficult to define or reproduce. But 

 radiology generally rejoices in a wealth of indefinite 

 units and measuring instruments, mostly introduced 

 bv workers who had enthusiasm but little physics. 

 The subject of practical radiology has, unfortunately, 

 been severely ignored by the physicist and the elec- 

 trical engineer. Their assistance in this matter is 

 earnestly needed both by the medical man and bv 

 the workers in this new field of the application of 

 X-rays to the examination of materials. The Rontgen 

 Society has on its roll of members most of the 

 younger X-rav physicists in the country, and we 

 suggest that it should take early steps to co-ordinate 

 the unrivalled experience and equipment of its phvsical 

 and medical members for the ultimate benefit of 

 British industrv. ' G. W. C. Kaye. 



RADIO-TELEGRAPHIC INVESTIGATIONS 

 IN CONNECTION WITH THE SOLAR 

 ECLIPSE OF MAY 29, 1919. 

 TT will be remembered that in a letter to Nature 

 ■*• of February 13 last Sir Oliver Lodge, chairman 

 of the British Association Committee for Radio- 

 telegraphic Investigation, explained that his com- 

 mittee did not contemplate taking a very active part 

 in organising wireless observations during the forth- 

 coming solar eclipse, and hoped that parties travelling 

 to the eclipse zone for the observation of astronomical, 

 meteorological, and magnetic phenomena might be 

 able to make wireless telegraph observations also. 

 Later it was found that the various parties charged 

 with the other observations would be too fully occupied 

 to give any attention at all to wireless telegraphy, 

 and therefore the committee has arranged for the 

 carrying out of the experiments to be described below. 

 The umbra intersects the earth's surface in an 

 approximate circle of diameter of about 234 km. 

 (126 sea-miles), and it moves at the slowest at a speed 

 of about 0-57 km. (0-31 sea-mile) per second. Between 

 11.30 and 12 (Greenwich mean time) it travels 

 across Bolivia and Brazil, and crosses the Atlantic 

 close to the equator between 12 and 14.20. It then 

 crosses the African continent from the French Congo 

 to Mozambique. During the eclipse various wireless 

 telegraph stations will emit signals consisting of letters 

 of the alphabet changed according to a definite plan 

 at the end of each minute ; the programme of letters 

 is so arranged that no two come together in the same 

 NO. 2584, VOL. 103] 



order more than once. They will be accurately timed 

 at selected _ receiving stations. By this arrangement 

 the transmitting stations are relieved of the responsi- 

 bility of timing the signals accurately, and the receiv- 

 ing operators have nothing to do but to write down 

 each letter as they receive it and the number denoting 

 its strength on the scale (o to 9) familiar to all wire- 

 less telegraphists. On the day before the eclipse the 

 stations will send practice signals for a short time 

 near noon (G.M.T.). 



The British Admiralty stations at Ascension and 

 the Azores will send continuously during the transit 

 of the umbra across the Atlantic Ocean. Observing 

 stations north of the equator will, for the most part, 

 be asked to listen to Ascension for at least an hour 

 round about the time when the umbra passes between 

 themselves and. Ascension. Similarly, observers south 

 of the equator will be asked for the most part to 

 listen to the Azores. Certain selected stations north 

 of the equator will be asked to listen to the Azores 

 so as to afford check observations upon the variations 

 that may be observed in signals passing across the 

 central line of the eclipse, and similarly selected 

 stations south of the central line will be asked to 

 listen to Ascension. The great American station at 

 Annapolis may also transmit a programme during a 

 portion -of the period of the eclipse, and it is hoped 

 that arrangements may be made for special experi- 

 ments between a few pairs of stations, such as Darien 

 and the Falkland Islands, and an Egyptian station and 

 a South African station. 



The main portion of the experiment hinges upon 

 Ascension. The umbral cone passes from west to 

 east, and may be expected to affect in succession the 

 strength in which signals are received at such stations 

 as Demerara, Jamaica, the stations on the coast of 

 the United States and Canada, and stations in Ire- 

 land, England," France, Italy, the Mediterranean, 

 and Egypt. 



It is by the kindness of the American Government 

 and of our own Admiralty that the stations at Darien 

 and Annapolis, and at Ascension and in the Azores, 

 are being used for the sending of the experimental 

 waves. The Admiralty has, besides, provided many 

 of the receiving stations both on land and sea, and 

 other receiving stations are being put to work by 

 the American, French, and Italian Governments, by 

 our own Army and Air Force and also by the Mar- 

 coni Co. in several parts of the globe. 



The observers' results will be collated with the view 

 of finding if the passage of the shadow cone between 

 a sending and a 'receiving station causes any regular 

 change in the strength of signals. According to some 

 writers, the propagation of waves over long distances 

 is greatly affected by the ionisation of the upper atmo- 

 sphere. During a solar eclipse the cone of densest 

 shadow removes all sunlight from the atmosphere 

 within it, which may stop the ionising actions of sun- 

 light and allow the recombination of separated ions 

 to take place. This process starts in the penumbra, 

 but it ig. accomplished fully, or to its fullest extent, 

 only in the umbra. Thus at any particular fixed 

 place in the air the penumbra, it is thought, first 

 starts a gentle recombination of ions, and as the eclipse 

 at that place progresses and darkness increases, re- 

 combination of ions takes place more ^ and more 

 quickly until the time of complete totality. After- 

 wards the onward passage of the umbral cone allows 

 sunlight to begin again its ionising action. Some- 

 thing of this kind is, at any rate, supposed to be 

 taking place at sunset and sunrise every day, and to 

 be the main cause of the enormous variations ex- 

 perienced in signal strengths at those times. 



It is sometimes supposed that the electric waves 

 carrving signals take a curved trajectory in the atmo- 



