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

well as standards of intensity. In some places, special constables were sent out and ordered the extinction of lights. Indeed, one-third of the observers in the inner area, and three-quarters of those in the outer area, at once assumed that a Zeppelin raid was in progress.

At several places within a few miles of East London, two reports were heard in quick succession. They were perhaps due to separate explosions. If so, the soundwaves of the second and slighter explosion soon faded beyond hearing, and the sound became a single crash or heavy thud. This character it maintained until the limits of the inner sound-area were nearly reached; and there, at a few places, the single sound was replaced by a double sound. Whether this change was general or not is unknown, for the sound-waves were even then preparing for their journey through the upper air across the silent zone.

When the sound-waves returned to earth once more within the outer area, all trace of the single sound had disappeared. Observers, it is true, occasionally speak of hearing only one sound. But this is clearly due to imperfect observation, for others at some of the same places heard two or three reports. The explanation, no doubt, is that the two reports occurred in quick succession, the interval between them being about half a second. Moreover, they were of nearly the same strength, so that the double report may well have seemed continuous. The same explanation probably accounts for the fact that two and even three reports were heard by different observers at the same place.

This explanation is confirmed by a map of the multiple-reports. It is then seen that double sounds were heard over nearly all the outer area, while triple sounds were also generally observed in this area, except near the western end, where the sound was distinctly fainter than elsewhere. Four reports were noticed only in the county of Norfolk-with one exception-and these were mostly within about seven miles of Norwich. In the neighbourhood of this city, observations of the sound were especially numerous, showing that the sound was probably louder there than elsewhere.

The interval between the second and third reports was about a second, and that between the third and