44 



NATURE 



[March io, 192 i 



tivity, with regard to which, I fancy, little is known. 

 For instance, with temperatures at which electrical 

 conductivity becomes infinite, does heat conductivity 

 also become infinite? It would seem that this can 

 be determined only experimentally. Perhaps Prof. 

 Onnes, or someone else who possesses the necessary 

 apparatus, could be induced to try the experiment. 

 Let us hope that someone will do so. 



A. A. Campbell Swinton. 

 66 Victoria Street, London, S.W.i, March 5. 



The Sound of Distant Gun-fire. 



The results of the comparison of observations made 

 on both sides of the fighting line upon the long- 

 distance audibility of gun-fire have been rather dis- 

 appointing. It appears that in Belgium and Germany 

 a very marked maximum was found everywhere in the 

 cold season, while in England and France the sounds 

 were perceived in the summer months only. More- 

 over, in the latter countries the guns were never 

 heard when the wind was blowing from the battle- 

 fields towards the observers, while in the former 

 the direction of wind seems to have been of little 

 importance. 



The vertical distribution of temperatures and varia- 

 tions of wind velocity with altitude are generally recog- 

 nised as the chief factors of the curvature of the trajec- 

 tories of sound, and they both bend the sonorous rays 

 upwards when temperatures are diminishing and the 

 strength of a head wind is increasing 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 charac- 

 teristic of air-flows, since, as a rule, friction against 

 the soil retards the lower strata. It appears, there- 

 fore, at once that the long-distance transmission of gun- 

 fire sound was observed in England and France when 

 the conditions favouring the bending upwards of the 

 rays were at their best. On the other hand, there 

 seems to have been nothing particularly favourable to 

 their being bent downwards in the upper air; first, 

 because in summer temperature inversions at moderate 

 heights are rare, and, secondly, because the con- 

 trary wind that was wanted was from between south- 

 west and north-west, and it is a well-known fact that 

 this wind generally occupies at all seasons the whole 

 height of the troposphere. True, its speed begins 

 usually to slacken above 11 km., and at about 20 km. 

 eastern components appear. But one might rightly 

 fear that rarefaction of the atmosphere at such alti- 

 tudes must already have reduced the intensity of sound 

 greatly. 



Now, on the German side it is quite the reverse; 

 the influences that curve the rays upwards are 

 at their lowest when the maximum of audibility 

 occurs, since this is the case in winter, when the 

 gradient of temperature is very feeble and often 

 reversed, and with indifferent direction of wind. But 

 these very Inversions are a powerful cause of bending 

 the ravs downwards. Thus with the ordinary wind- 

 temperature theory we cannot escape an almost com- 

 plete contradiction. 



The hydrogen-atmosphere theory of van den Borne 

 and van Everdingen cannot help us out of this per- 

 plexity, since in this theory the long-distance per- 

 ceptibility of sound should be quite independent of 

 meteorological conditions, not to speak of the insuper- 

 able difficulty of attributing sufficient intensity to a 

 sound travelling through a vacuum of 001 cm. at 

 70 km. 



I therefore think there Is only one way of escape, 

 namely, tb advocate diffraction. It is well known 

 that sonorous rays are endowed with this property in 



NO. 2680, VOL. 107] 



a remarkable degree, and along such flat trajectories 

 as must be the case in the long-distance propagation 

 of sound, refracted rays cannot fail to diffuse to the 

 earth all along. It is rather surprising that there 

 should be a silent zone at all. Now, in ordinary cir- 

 cumstances these refracted rays, coming back to earth 

 in all directions from the source, would be too faint 

 to be perceived by any but a very attentive and well- 

 trained ear as soon as a moderate distance from the 

 centre of emission is reached. Should, however, any 

 cause productive of upward curvature bend the rays 

 that make a small angle with the horizon, then a 

 caustic will be formed by these rays, and also by the 

 diffraction rays issuing therefrom, so that the intensity 

 of sound in these bundles of diffracted rays will grow 

 sufficiently for hearing to be possible. The causes of 

 upward bending, viz. vertical gradients of decreasing 

 temperature and decreasing force of wind, are as a 

 rule the more marked the nearer the earth one con- 

 siders them. Thus the rays nearest the horizon are 

 the most energetically bent, and the whole group inter- 

 sect one another at small angles, thus forming beams 

 where intensity is at a maximum. 



In this theory diffraction would be the normal cause 

 of the return to the earth of the sonorous waves in 

 England and France ; temperature gradient and con- 

 trary wind would only have to concentrate the rays 

 in caustic bundles in order to intensify the sound at 

 great distances. If temperature inversions and 

 change of wind velocities or directions add their 

 influences in order to bend the sound-tracks down- 

 wards, as in the German winter conditions, the direct 

 rays themselves might be deflected towards the earth. 



In this way everything seems to have a satisfactory 

 explanation except the summer minimum of Ger- 

 many. This is a very remarkable feature indeed, and 

 very perplexing, for in summer as well as in winter 

 the conditions for the return of the sound rays seem 

 to be altogether more favourable on the German 

 than on the Anglo-French side. For over the con- 

 trary east winds that bend them upwards flow, as a 

 rule, the permanent west currents of the higher tropo- 

 sphere, the effect of which is to bend them down. 

 One might wonder whether, perhaps, their bending 

 effect is not too strong, and whether all but the ravs 

 damped by their passage through highly rarefied air 

 are not brought back to earth too soon for a long- 

 distance audibility zone to be possible ! This hypo- 

 thesis seems worth examining closely. 



At any rate, the problem has lost Its pleasing sim- 

 plicity, and there Is little hope that observations made 

 during the war and not yet published will solve it 

 adequately. One thing, therefore, remains to be done, 

 and that is to turn to that supreme criterion — experi- 

 ment. 



Now this means organisation with vast resources 

 and on a huge scale. Batteries should be fired on 

 some suitable spot of the ancient Front (to facilitate 

 taking into account the observations of the war) 

 and observers posted along well-chosen lines, chiefly 

 in the directions against and with the wind, at various 

 distances in the air as well as on the ground. The 

 salvoes should be fired at pre-arranged hours, so as 

 to permit of calculating the trajectories travelled 

 through by the reports. At the same time, and about 

 the same places, meteorological observations as com- 

 plete as possible should be made, and they, too, should 

 be takeri by aeroplane and dirigible at all suitable 

 heights as well as on the earth. 



No doubt this would be a tremendous business. 

 But let It be remarked that there was a long period 

 of time when It could have been done with little cost 

 and scarcelv any difficulty ; this was in the months 

 following the conclusion of peace, when Immense 



