Art. 3.-THE SOUND OF A GREAT EXPLOSION.

ON Jan. 19, at 6.51 p.m., a great explosion-perhaps the greatest ever experienced in these islands-occurred at an important munitions factory in East London. The factory in question is well known, and it will therefore be sufficient if its position be represented roughly by the star in the accompanying map.

The interest of the explosion from a scientific point of view is indirect and lies in the evidence which it offers on the mode in which sound travels through the atmosphere.* In this explosion, as in so many others, the area over which the sound was heard is not continuous; it consists of two detached portions. One portion, of course, surrounds East London. It will be convenient to call it the inner sound-area.' The other lies far away to the north, and may be known as the 'outer soundarea.' Between the two, there lies a broad zone of silence,' in which, with one exception, all trace of the sound seems to have been lost. There is nothing novel in this fact. It has been well known since the beginning of the century, when the minute-guns fired during Queen Victoria's funeral procession revealed the existence of a silent zone for the first time. Many subsequent explosions, both artificial and natural, have exhibited a similar zone. The peculiarity of the recent explosion is that it occurred in, and manifested its effects over, a populous district, from which a large number of observations could be easily obtained.

The continuous lines in the accompanying map bound the two areas over which the sound was heard. They depend on an unusually large number of observations (571 from 391 places). It was essential, however, to show that the intermediate district was really free from all trace of sound, and I therefore made inquiries at many places within the silent zone and especially at those close to the boundaries as I had drawn them. Similar letters were also sent to places just outside both sound-areas. The position of these places (71 in number) aided materially in the exact delineation of the boundaries.

'Cf. the article on 'The sound of big guns' in the Q. R. for July 1916.

The inner sound-area, as will be seen from this map, is of rather unusual form. It shows a marked extension in two directions, one towards the east-south-east, terminating near Canterbury at a distance of 48 miles from the source of sound; the other running in a northwesterly direction through Hertfordshire and Bedfordshire, and ending just beyond Northampton after a course of not less than 66 miles. In comparison with its distortion along these two lines, the restriction of the sound-area in other directions is all the more remarkable. Towards the north-east and south, the boundary passes through points which are only 20 and 19 miles respectively from the origin. The total area included within the inner boundary curve is 3390 square miles.

Of very different form is the outer sound-area. It stretches in a broad band, in a direction a little south of east, from the neighbourhood of Nottingham to, and no doubt slightly beyond, the east coast of Norfolk. Its length, as far as the coast-line, is 131 miles, its greatest width is 44 miles, and the area covered by it is 5000 square miles, or half as large again as the other. Near its northern boundary lie the most distant places at which the sound was certainly heard-Bestwood (near Nottingham) and Stow (near Lincoln), which are respectively 117 and 128 miles from the origin.

For a great explosion, these distances are by no means excessive. Possibly, the amount of explosives lost was not so considerable as in other explosions; but on this point we cannot expect to be enlightened. At any rate, the distances are smaller than that attained by the soundwaves of the Wiener-Neustadt explosion on June 7, 1912, when 197 tons of gunpowder exploded and the sound was heard to a maximum distance of 186 miles.

The observations which I have received from the two sound-areas point to great variations in the loudness of the sound. The epithets used show that the sound may be readily divided into three grades of intensity. In the highest (A), the sound was evidently very loud. Such terms are used as 'a terrible explosion,' or 'louder than any bomb or crash of thunder.' In the next (B), the sound was moderately loud, as is clear from such expressions as 'a loud report,' 'heavy gun fire' or 'a heavy clap of thunder.' To the lowest grade (C), that of faint

sound, correspond such terms as 'distant guns booming,' the distant roll of thunder,' or 'a heavy gust of wind.'

The scale of intensity is clearly a rough one. Yet it is remarkable how closely the places corresponding to each degree of the scale are grouped together-so closely, in fact, that curves may be drawn separating places of one grade of intensity from those of another. These curves are represented by the dotted lines on the map. In the inner area, there are two such lines of equal sound-intensity (A and B) including the places at which the sound was very loud and moderately loud, respectively. In the outer, there is only one such line (B)—that of moderate strength. In each case the boundary includes all places at which the sound was faint.

There are two points of interest brought out by these curves. The first is the rapidity with which the intensity declined outwards in the inner area. The curve of highest intensity (A) includes only 104 square miles, that of moderate intensity (B) 950 square miles. Thus, the district included between the curve B and the boundary is nearly three-quarters of the whole inner area. It does not follow that the sound-waves really lost so much of their intensity after traversing distances so short. It is rather that, before starting on their journey over the silent zone, the sound-waves had already begun to rise, and that only secondary waves, as it were, spread out from their lower fringes so as to reach observers on the ground.

On the other hand, the intensity over the outer area was much more uniform. In no part of it was the highest degree attained, but the curve of moderate intensity covers a district measuring 2740 square miles or nearly three times as large as the corresponding part of the inner area. If we consider that the central portion of the outer area is nearly one hundred miles from East London, this surely is a strange result. And no less remarkable is the fact that, in more than one-half of the entire outer area, the reports were louder than over nearly three-quarters of the inner sound-area.

Between the two areas lies the mysterious silent zone. How much it varies in width is evident from the map. At the western end, the zone is 28 miles wide; at the eastern end, no less than 48 miles. All over this zone,

but for one place, the sound-waves crossed unheard. The exceptional place is Ipswich, where a sound was heard by one, and, so far as I know, by only one, person. There is no reason for discrediting the observation, simply because it is exceptional; and, in any case, the place is only ten miles from the boundary of the outer soundarea; and the observation was made in the highest part of the town. Again, in the narrow neck at the western and more lofty end of the silent zone are two places (Uppingham and Lilford), from which records of the sound come. But the observations at these places are not free from doubt, because the estimates of the time at both differ by about a quarter of an hour from the correct value, though the difference is within the error of many country clocks. There are, however, observations of another kind from this district. These will be referred to on a later page, for they have an important bearing on that difficult problem-the origin of the silent zone.

Again, from the country lying to the north, west and south of the sound-areas, there have come a few reports. Some of these deserve attention because the observed time does not differ widely from that which might be expected, taking into account the known velocity of sound. The observations, however, are unsupported by others in the vicinity, and the places are frequently reached by sound-waves from other distant sources. Thus, while we may surmise that the sounds may have resulted from the explosion in East London, we cannot regard their connexion with the explosion as proved.

All over both areas, the sound was a deep boom, so low that, according to several observers, it was almost more felt than heard. It is a remarkable fact, and one worthy of the attention of physiologists, that the sound was certainly heard by a man who is deaf to all but the loudest shouting. As a rule, the sound was merely a crash or heavy thud. In some places, it was followed immediately by a rumbling like that of thunder and probably due to the same cause. In the outer area, though a rumbling was observed occasionally, the sound was usually sharp and brief. Enemy air-ships have often visited both areas; and the sounds of their exploding bombs have been of service as types of comparison as