July 13, 1916] 



NATURE 



403 



than 100 km., and the intensity of the sound at 

 this least distance is not less than near the 

 boundary of the inner sound-area. 



Dr. van Everdingfen refers to several dynamite 

 -and volcanic explosions which have been carefully 

 studied from 1903 to 191 1, He also adds some 

 interesting- observations made chiefly in Holland 

 during- the present war. The most important 

 case is that of the bombardment of Antwerp on 

 October 8, 1914. The reports were heard at 

 many places in Holland within 100 km. from the 

 source and ag-ain outside a circle of 158 km. 

 radius, but at very few intermediate places. The 

 .silent region is bounded by two curves, which 

 are roughly circular, the inner arc being traced 

 for more than 180° and the outer for more than 

 •90°. In some cases of heavy firing- at later dates 

 there are also indications of silent regions ; in 

 others an increased audibility has been established 

 near the line of 160 km. In no case is there any 

 certain indication of any asymmetrical propa- 

 g-ation of the sound. ^ 



Dr. van Everding-en examines the two exolana- 

 tions which have been offered of the existence of 

 the silent region, one of which relies on varia- 

 tions of wind-velocity and temperature with the 

 altitude; the other (von dem Borne's) on changes 

 in the composition of the atmosphere at great 

 heights. On the former explanation we might 

 expect asymmetry, on the latter symmetry, with 

 regard to the source of sound. He considers that 

 both explanations are true and should be applied 

 in combination. In favour of the second explana- 

 tion, he urges the facts that in recent cases the 

 outer margin of the silent region has always been 

 about 160 km. from the probable source of 

 sound and that no appreciable deviations from the 

 circular form have been observed. The above 

 distance is greater than the limiting- distance 

 (114 km.) assigned by von dem Borne, but Dr. 

 van Everdingen shows that it agrees well with 

 estimates made on the supposition that the per- 

 centage of hydrogen in the upper atmosphere is 

 much smaller than that assumed by von dem 

 Borne. 



There can be no doubt as to the value and in- 

 terest of Dr. van Everdingen 's investigations. It 

 would seem desirable, however, to continue and 

 extend them. Though the existence of silent 

 regions may be reearded as established, many 

 more negative records are required to prove the 

 symmetry of the region with reference to the 

 source of sound. It must be remembered that 

 the deep sounds of these explosions may at great 

 distances be below the lower limit of audibility 

 of some observers. Moreover, the mean radius 

 of the outer margfin of the silent region is very 

 far from being- constant. In one of the earliest 

 cases in which the silent region was noticed — that 

 of the minute-guns fired during the funeral pro- 

 cession of Queen Victoria on February i, 1901 

 {Knowledge, vol. xxiv., 1901, pp. 124-5) — ^^^ 

 radius was about 80 km. C. Davison. 



2 It may be mentioned that, on October 28, 1914, the sound of the British 

 naval guns that bombarded the Flemish coast was heard at a distance of 

 280 km., or 174 miles. 



NO. 2437, VOL. 97] 



AERONAUTICS AND THE WAR.^ 



(i) |V/[ R. LANCH ESTER'S latest book, unlike 

 ^^^ his previous works on aerial flight, can 

 be read with considerable interest and without any 

 great effort. The preface, by Lieut. -General Sir 

 David Henderson, at once arrests attention and 

 has caused more comment than any other equally 

 long section of the book, i'he summary of the 

 present aeronautical position is so interesting that 

 a quotation of considerable length is here given. 

 General Henderson writes : — 



There are no experts in military aeronautics ; there 

 are experts in the various branches : in flying, in 

 scientitic research, in the design and construction of 

 aeroplanes and engines, in military organisation and 

 tactics. But as yet there is little opportunity for the 

 expert in one branch to gain definite knowledge of 

 the others except by hard personal experience; in 

 every direction there is progress, in ever\- section of 

 work opinion is fluid. ... Of all the fields in which 

 work for the advancement of military aeronautics has 

 been undertaken in this country, that of scientific 

 research has, up to the present, produced the results 

 that will probably be most enduring. ... In the 

 work of stating and solving the problems of aero- 

 nautics, Mr. Lanchester was one of the pioneers; he 

 was bold enough to publish the results of his investi- 

 gations at a time when flying had only just been 

 proved possible; and he has reason now to be well 

 satisfied with the quality of his early work. 



The author himself, in his introductory note, 

 rubs in the last point very vigorously. 



Mr. Lanchester commences by describing the 

 functions of an aeronautical arm, stating that 

 reconnaissance is the main duty, in which aircraft 

 are related to the older arms of the Service. The 

 opposing and . destruction of enemy aircraft are 

 classed as secondary functions. The problem of 

 the relative merits of aeroplane and dirigible is 

 treated at some length. Attention is directed to 

 the superior speed of the aeroplane (practically 

 double that of the dirigible). The limit of size is 

 practically reached for the dirigible, whereas the 

 present-day aeroplane nowise defines the limit, in 

 Mr. Lanchester's opinion. This seems scarcely 

 consistent with his present views, for his recent 

 article in Engineering expresses the opinion that 

 large aeroplanes will be less efficient than smaller 

 ones. Mr. Lanchester is doubtful whether fight- 

 ing is a primary function of the dirigible, and 

 thinks that bomb-dropping is altogether a misuse. 

 He points to the vulnerability of the airship, stat- 

 ing that "even to-day the finest of Germany's 

 fleet of Zeppelins would be absolutely at the mercy 

 of a modern aeroplane in the hands of a man pre- 

 pared to make his one and last sacrifice." Before 

 proceeding to more general considerations he dis- 

 poses of the dirigible as a part of the aeronautical 

 ser\'ice, pointing out that if this proves untrue his 

 main conclusions will not be affected. 



The question of the vulnerability of the aero- 



1 (1) "Aircraft in Warfare: the Dawn of the Fourth Arm." By F. W* 

 Lanchester. Pp. xviii + 222. (London : Constable and Co., Ltd., 1916.' 

 Price I2J. M. net. 



(2) "Aircraft in War and Peace." By W. A. Robson. Pp. xi + 176. 

 (London : Macmillan and Co., Ltd., 1916.) Price zr. 6rf. net. 



