140 



NATURE 



[March 31, 192 1 



The Sound of Distant Gun fire. 



With reference to the letter of Father Schaffers 

 in Nature of March lo, it is certainly a fact that 

 sounds from moderate distances are heard most plainly 

 when there is a wind reversal at a moderate height 

 and when the upper wind comes from the same direc- 

 tion as the sound. At this place the sound of firing 

 off the east end of the Isle of Wight is heard best 

 when a south wind is blowing over a light wind 

 from some other quarter. As regards conditions 

 when the sound of gun-fire from the Front was 

 heard in this country, I do not altogether agree with 

 what Father Schaffers writes. He says that sound- 

 waves are bent upwards "when temperatures are 

 diminishing and the strength of a head wind is in- 

 creasing with altitude. The former is at its maximum 

 efficiency in summer, when there is a steep gradient 

 over the surface of the earth ; the other is nearly 

 always a characteristic of air-flows, since, as a rule, 

 friction against the soil retards the lower strata." 



Father SchafTers goes on to say that temperature 

 inversions at moderate heights are rare in summer, 

 and that at all seasons a wind between south-west 

 and north-west — that is, a head wind for sounds 

 coming towards this country from Flanders — generally 

 occupies the whole height of the troposphere. But 

 with anticyclonic weather and with easterly surface 

 winds these conditions are not always realised, and 

 I 'am under the impression that it was chiefly in such 

 weather that the sounds were best heard. There are 

 certainly many occasions when the temperature 

 gradient is very slight in clear, anticyclonic weather ; 

 and in an easterly wind there is often a sharp increase 

 of velocity up to i km, or 2 km. before any decrease 

 takes place. Moreover, it often happens in summer, 

 and in other seasons, that no westerly wind is met 

 with at any height up to the top of the tropo- 

 sphere. 



There are, therefore, it seems to me, many occa- 

 sions when a sound-wave might be refracted down- 

 wards by an easterly wind and reach the surface a 

 considerable way to the west of the source. Sound- 

 waves that went up at a fairly high angle might get 

 through the strongest part of the easterly wind and 

 never reach the surface, but those which went up 

 at a less angle would be refracted and never get 

 through the easterly wind. I am inclined to think that 

 any cause which occurs, to make sounds to be heard 

 at great distances must operate fairly low down in 

 the atmosphere; if the waves went to a great height 

 before being bent down the sounds would seem to 

 come from high up, whereas my experience was that 

 they seemed to come from somewhere near the 

 horizon. If this is the experience of others it should 

 rule out the hvdrogen-atmosphere theory ; a sound-ray 

 which went up to 100 km., say, and was thence 

 refracted down to the surface at a distance of 200 km. 

 from the source would come down at an ancle of 4::°, 

 and such sounds would hnve been attributed by 

 ordinarv observers during the war to some aerial 

 fighting. 



The question of the propagation of sound-waves in 

 the atmosphere has been very fully dealt with by Mr. 

 S. Fujiwhara (Bulletin of the Central Meteorological 

 Observatory of Japan, vol, it,, Nos, i and 4). Mr. 

 Fujiwhara maintains that the abnormal propagation 

 of sounds to great distances, silent regions, and 

 regions of double audibility depend on the wind struc- 

 ture of the atmosphere, and that sound-waves mav be 

 reflected in certain conditions of a heterogeneous wind 

 structure. He has taken certain cases of wind struc- 

 ture revealed by pilot-balloon ascents at Ditcham, and 



NO. 2683, VOL. 107] 



has calculated theoretically the regions of audibility 

 which should be found under the conditions existing 

 at the time ; he finds that these agree fairly well 

 with the size and shape of the areas of audibility of 

 explosions of the volcano of Asamayama. He also 

 maintains that the wind structure of the atmosphere 

 at the time of an explosion may be deduced from the 

 areas of audibility. 



C. J. P. Cave. 

 Ditcham Park, Petersfield, March 21. 



Sound Transmitted through Earth. 



The letters from Mr. C. Carus-Wilson and Dr, 

 Charles Davison in Nature of March 24 prompt me 

 to give the following experience 



In June, 1903, I was trekking towards the Victoria 

 Falls. On the night before arrival we " outspanned " 

 some twelve miles to the south, and on retiring to rest 

 on the bare ground I became aware of a curious, 

 rhythmic sound, quite distinct when my ear was 

 pressed against the soil. I told my two brothers, 

 who found they also could hear the pulsation, and 

 one of them suggested that it must be due to the 

 booming of the distant cataract. 



To me the most interesting point is not that th^ 

 sound was transmitted by the earth, but that it was 

 transformed into rhythmic vibration — very difTerent 

 from the constant roar one hears when close to the 

 Falls. Some process of interference would seem to 

 occur and give rise to this result. 



Reginald G. Durrant. 



Rosetree, Marlborough, March 26. 



X-rays and their Physiological Effects. 



The death of my brother. Dr. Ironside Bruce, from 

 a hitherto unsuspected danger in the use of X-ravs 

 by medical men for purposes of treatment and 

 diagnosis has an aspect other than its personal or 

 medical one. I only write to Nature because I feel 

 impelled to address an appeal to workers on the purely 

 physical research problems connected with X-rays. 

 I suggest that there is a need for closer association 

 between the latter and medical men practising radio- 

 logy. The advance in medical knowledge which the 

 X-ray has rendered possible has been immense, 

 and it is becoming practically indispensable in the 

 diagnosis of disease. But it is now clear that its 

 use by practitioners may be curtailed unless some 

 more effective measures of protection for radiologists 

 can be devised. 



On many occasions my late brother expressed to 

 me his difficulty in obtaining precise physical know- 

 ledge bearing on the nature of the rays and their 

 effects on human tissues. Not many days before his 

 death he returned to this subject, and said that if he 

 recovered he would devote his life to research on pro- 

 tective measures. If a layman might venture an 

 opinion, it would be that medical men generally can- 

 not be expected to conduct research on the methods 

 of production of the ravs, or on the exact nature of 

 the various kinds of rays produced bv difTerent forms 

 of apparatus. On the other hand, physicists are not 

 ordinarilv competent to investigate the purely 

 biological effects of the rays. Hitherto medical 

 men have been lulled into security by the belief 

 that the only injury to be feared was dermatitis, 

 which thev believe is caused by rays of low 

 "penetration," and are probably stopped even 

 bv ordinarv clothing materials. Again, they believe 

 that protective screens of lead ijiass afford full pro- 



