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the slight spreading of the lower portions of the waves. Thus, the comparative inaudibility of sound-vibrations in the direction from which the wind comes is due not so much to any enfeeblement in the vibrations as to their being lifted over the heads of observers on the surface of the ground.
If the wind be travelling in the same direction as the sound, the actual velocity of the sound-wave on the same suppositions would be 1110 feet per second on the ground and 1120 feet per second at eight feet above it. In Fig. 2,
the sound-wave is again supposed to travel from left to right, and the wind is now supposed to be in the same direction. The lines indicate the positions of the wavefront at successive equal intervals of time, on the same scale as in Fig. 1. The result is thus to tilt a wave-front that was initially vertical in a downward direction; and also to bring sound-waves that started on an upward course after a time towards the ground. The propagation of sound-waves to a great distance with the wind is therefore mainly due to the fact that sound-waves, that would otherwise be lost in the upper air, are deflected downwards to observers on the ground.
The explanation of the curious anomaly noticed during the funeral procession of 1901 will now be obvious. In the neighbourhood of Spithead, the sound-waves were first lifted by contrary winds over the heads of observers at the level of the ground. Afterwards, by favourable winds, they were brought down to that level, so as to be distinctly audible to persons stationed at distances of from 50 to 139 miles.
Variations in wind-velocity, however, are not the only cause of the anomalies described above, and they furnish no explanation of the distant propagation of the reports
of the firing at Cherbourg in 1900. The variations in temperature here come into play. It is well known that the velocity of sound in air does not depend on the actual barometric pressure at the time, but varies distinctly with the temperature. With a rise of 1° C. in the temperature, the velocity of sound increases by about two feet per second. Now, during the day the temperature of the air usually decreases with increasing height above the ground; and thus, in all directions from the origin, the sound-waves tend to rise above the ground, just as they do with a contrary wind. This is one of the reasons, but not the only one, why sounds of given intensity are less audible at a distance on a hot summer's day than at other times. If, however, the temperature were to increase with the height above the ground-and this is known to occur at sea shortly after sunset_the effect would be the same as with a favouring wind; but, the cooling of the air being confined to a thin layer, the sound-waves, as during the Cherbourg review, would be audible to great distances in the immediate neighbourhood of the sea-level.
The variation in the wind-velocity and temperature with the height above the ground is of the simplest possible character in the cases here considered. With such variations there can, as Mr S. Fujiwara has shown,* be no silent regions and no repetitions of the sound. A more complicated law of variation is, however, conceivable. The velocity of the wind, for instance, may increase with the height up to a certain point and then decrease. Under such conditions, it may be proved that
egions or islands of silence must exist, and that the sound-waves may reach a distant station on the earth's surface by two or even three paths of different lengths, so that a brief sound, like that of the Asama-yama explosions, may be heard twice or thrice at one place.
* 'On the abnormal propagation of sound wave in the atmosphere': Bull. of the Centr. Meteor. Obs. of Japan,' vol. 2, 1912, pp. 1-143.
† Since this article was written, an important paper by Dr E. van Everdingen has been published. He describes several cases of recent gunfiring heard to great distances in Holland, in one instance to 174 miles. In some cases described by him, there is evidence of the existence of a silent region, which he attributes partly to variations in wind-velocity, partly to changes in composition which are assumed to occur in the upper atmosphere. Vol. 226.-No. 448.
There remains to be considered an incident closely connected with the subject of this paper. On Jan. 24, 1915, a Sunday morning, there was a running fight in the North Sea between the First Battle-Cruiser Squadron under Vice-Admiral Sir David Beatty and the German cruisers Derfflinger,' Seydlitz,' Moltke,' and Blücher,' and other minor vessels. The · Blücher,' as is well known, was sunk during this engagement. The position of the vessels during the action has not yet been made public, but they must have been some distance from the shore before our ships came within range of the enemy, for, while the sound of the firing was heard near the Lincolnshire coast, nothing but a 'soughing in the ear' was observed about one hundred miles farther inland at Ripleth, near Ripon. During the battle, from about 10 to 11.30 a.m., there was much agitation among the pheasants in various parts of the north of England. According to the parish-clerk at Saxby in Lincolnshire, • There be rare goings on in the North Sea the morn;.. the pheasants is all over the place with their fuss ;' and his remark was made before the news of the battle arrived. Similar observations were made in various parts of Yorkshire, at Lowther near Penrith, and even at places in Cumberland which are probably 200 or 250 miles from the scene of the firing. There can be little doubt as to the close connexion between the gun-firing and the disturbance of the pheasants, for, in woods near Burgh-le-Marsh in Lincolnshire, the firing and the crowing of the pheasants were heard together.
In what way are pheasants affected by the distant gun-firing? Do they actually hear sounds which are too deep or too faint to produce any effect on the human ear? Or is it that they are in some way susceptible to the evanescent air-vibrations or are alarmed by movements due to those vibrations ?
We know, indeed, very little about the varying capacity of the human ear for appreciating the low roll of distant gun-firing. We know still less about the powers of birds and animals for hearing such vibrations. The only evidence with which I am acquainted is their behaviour during earthquakes. For instance, during the Hereford earthquake of 1896, pheasants crowed at a distance of 111 miles to the north-west of the origin; the
sound was heard to a distance of 170 miles in the same direction. During the Doncaster earthquake of 1905, the farthest place at which pheasants were affected is 38 miles from the origin; the sound was heard on an average for 62 miles from that place. The evidence is not quite conclusive, for pheasants are not so uniformly distributed as human beings over the country. So far as it goes, however, it seems to show that the pheasant's ear is less sensitive than our own to very deep sounds.
On the other hand, it must be remembered that even human beings are affected by sound-waves in other ways than through the ear. When there is a loud report close at hand we instinctively wink. It is the reflex action of the eyelids to protect the eyeballs from injury when the air-waves suddenly impinge upon them. It is possible, indeed, that pheasants never hear the report of guns at all, however close they may be, and that it is merely the resulting air-vibrations striking on their bodies that alarms them. On the whole, however, it seems more probable that the air-waves act only indirectly on the birds. We have seen that the reports of the guns during the Cherbourg review were heard for 107 miles, but that for 30 miles farther the airvibrations were strong enough to make windows shake and rattle. In the same way, far beyond the Lincolnshire woods in which the guns were heard on Jan. 24, inaudible waves would speed their way across the country. During their passage low trees and undergrowth would suddenly sway and quiver. The birds resting on them would be alarmed by the abrupt though slight disturbance, and would rise with the excited cries which they utter when somewhat similar movements are caused by the passage of earthquake waves.
Art. 13.-THE COURSE OF THE WAR ON LAND.
Both in the various theatres of operations, and in the field of politics connected with the war, the past three months have been marked by notable events. The period has been one of exceptional activity on the part of the enemy, which seems to denote a desperate effort to attain a favourable decision during the current campaigning season. As was expected, for reasons given in the last article, the Germans have continued to devote their energies to the western theatre of war. The Austrians have massed a considerable force in the Trentino for the invasion of Italy. The bulk of the Turkish army has been transferred from the Near East and from Syria to oppose the Russian and British forces in Armenia and Mesopotamia; and the Bulgarians, by seizing the Rupel Pass, and by hostile demonstrations at other points in Macedonia, have caused General Sarrail some embarrassment, and created fresh friction between the Greek and the Allied Governments. The abortive insurrection in Ireland and the naval adventure in the North Sea, though outside the strict limits of our subject, must be mentioned incidentally as forming part of a comprehensive scheme for weakening the Allies by land and sea, and for keeping their forces dispersed, by which the Germans hoped to facilitate the attainment of their main purpose.
But the Allies, on their side, have not been idle. The Russians had devoted themselves to the reorganisation of their arrangements for the supply of men and munitions for the army, with a success which enabled General Polivanoff, the War Minister, to declare, on Feb. 8, that a permanent reserve of a million and a half young recruits had been assured, and that the munition crisis no longer existed. There was reason to hope that they would be able to take the offensive when climatic and other conditions should be favourable, a hope which recent events seem to have justified. The expansion of the British army, and the calling-in of some troops which had become superfluous in other regions, had enabled Sir Douglas Haig to occupy the entire line from Ypres to the Somme, thereby releasing a large force to augment the French reserves, and to assist in the operations about Verdun. This extension of the British front in France,