sound refraction is specially noticeable. We are deaf, so to speak, to the sounds on one side of us. This phenomenon is of course perfectly familiar, and was explained mathematically by Osborne Reynolds some fifty years ago. Interest in the matter has revived since listening to aeroplanes has brought it into prominence. With the more perfect meteorological data now available we are able to anticipate good hearing, and from a knowledge of the upper winds of which the Meteorological Office keeps us informed we may estimate the amount of refraction and hence the listening ranges. One of two illustrations may perhaps be of interest. The one o'clock gun which was for so long familiar to the inhabitants of Woolwich furnished a source of sound of remarkable constancy. In temporary buildings not 600 yards away the sound of the explosion was heard and used as a signal for the lunch-hour break. On some occasions the explosion produced an unpleasant shock; on others so little sound was received that the signal was actually overlooked. The gun was visible from the place referred to, and the effect was entirely one of refraction. By simple calculation it could be shown that the explosion would be heard with almost uniform intensity from day to day at the top of the neighbouring wireless mast, 80 feet high. One may regard the gun as being thrust below the acoustical sky-line for the observer on the ground. If the observer ascends he can extend the range of his acoustical horizon -just as from a kite-balloon an observer can get extended visual range. Let the listener take note of traffic on the road and observe the range of hearing a car when approaching and receding, or let him observe a railway train under the same conditions. The aural time of warning of approach, which may have some importance, is a measure of the amount of atmospheric refraction of sound. In a cross-wind the ranges would be equal.

On a still night abnormally short or long ranges are sometimes perceived. This is due to refraction also, the refraction being caused by variation in temperature of the different air strata. If the lower layers of air are colder than those above, the sound travels most favourably along the ground. One illustration may be quoted.

On a calm evening in the autumn the temperature of the waters of the Exe estuary would be cooler than the air above it, and the resulting temperature distribution would be such that the lower air strata would be cooler than those above. This is opposite to the normal distribution, which gives a gradual fall of temperature on ascent. To the listeners on the Exmouth front the sounds of the Great Western trains proceeding to Dawlish would then be remarkably loud, giving a surprising sense of nearness to a track which is at least one and a half miles away.

Perhaps, however, the evidence of refraction is more noticeable when listening to approaching aircraft. When listening to an approaching aeroplane flying against the wind it is surprising how suddenly the sound announces the approach. It has been shown that the aeroplane climbs over an acoustical sky-line,' and we get the dawn of a diffused sound, hard to locate with accuracy, suddenly developing into the full effect when the source of sound has 'risen.' Aural sense of direction then becomes perfectly definite.

Before climbing the acoustical sky-line there is the acoustical dawn,' or for the reverse direction the acoustical twilight,' the duration of which depends on the acoustical transparency of the air. We borrow the phraseology of light in order to give a conception of these acoustical effects. The duration of twilight in our vapour-laden atmosphere extends to nearly an hour; in tropical climates this duration is short. So also in atmospheres laden with acoustic clouds' or 'acoustic dust' we get the extension of time during which the sound of an approaching aeroplane is diffuse.

These acoustic clouds' are whirls and eddies, patches of warm or cold air, masses of air more or less humid, but possibly quite invisible to the eye. On a warm summer day there is never freedom from these clouds, on a calm summer evening the clouds dissipate. We hear better, not only observing a greater loudness of the sound, but also acquiring a keener sense of direction.

The acoustical atmosphere' can vary so greatly that the thunder peal in the greatly disturbed air is lost at a few miles, while on other occasions the relatively weak sounds of wheeled traffic can be heard for the same range. On a hot, oppressive day the hum of insects quite near the listener seems insistently loud, while he is conscious of a blanketing of distant sounds. Like the glare of the headlights thrown back on the driver of a car moving in mist, the sound is thrown back, and at the same time, like the blurred faintness of the headlights of an approaching car, the distant sound appears abnormally weak.

In patchy fog the steamer siren gives a blast whose reverberations continue for several seconds to the listener on the ship. The range is then small; yet if the fog is uniform the acoustical atmosphere' is clear, the range is long, and the sense of direction good. At the same time the reverberation as observed on the ship is very small.

Another effect due to absorption by these 'acoustic clouds is the change in character of the transmitted sound. If produced by a very distant source like the siren of a light-vessel, the sound is heard with so marked a variation of loudness that the perception of range is constantly varying. A patch of acoustic cloud may

temporarily extinguish the sound, and between blast and blast at · intervals of a minute, and at a range of seven or eight miles, the intensity frequently varies from ten to a hundredfold, as has been shown by instrumental measurements. A continuous sound like that of an aeroplane at that range becomes apparently intermittent. Not only in loudness, but also in quality, sound may change under such conditions. Just as the setting sun shows a redness through absorption of long stretches of dust and vapourladen atmosphere, so also may the complex sound of the siren be deprived of its high-pitched constituents-the raucousness of the sound is diminished, and even the most unpromising Klaxon horn may show some evidence of musical character.

An experiment was performed with a large concave concrete mirror with a Klaxon horn in the focus, and ranges of seven miles were obtained with the resulting sound beam. The sound emerged as a musical note. The distant aeroplane may hum like an organ pipe, and the Klaxon horn which it may carry for soundsignalling purposes may coo' with a character quite unrecognisable. For this reason Trinity House employs the low-pitched siren rather than the high-pitched whistle.

The distant band, the far-away peal of bells, become mellow by the absorbing properties of the atmosphere. The low notes of the drum are the last evidences of receding music.

Experiments in listening range have indicated the limitations of sound for sound-signalling purposes. Fortunately for the listener, developments in 'wireless' ranges have not been paralleled in sound. Although very intense sources of vibration can be produced, the atmosphere shows itself incapable of conveying these vibrations away in the form of sound without great losses in energy. The multiplication of horse-power in a soundproducing installation gives little advantage. The atmosphere, therefore, protects us from extreme discomfort. We have also a very remarkable power of accommodation in our listening mechanism, so that we can automatically render ourselves insensitive to intense sounds while still preserving, as occasion demands, our power of hearing and analysing the faintest sounds.

Nevertheless, some attempt should be made to protect the ear and the nervous system involved in the operation of listening from the excessive noises which modern traffic and conditions impose upon us. The faculty of listening will be blunted by over-exertion, just as the eye can be dazzled and rendered inefficient by too strong a light.

There is no more wonderful instrument than the human ear, and one may, in conclusion, urge the importance of its claims for education in critical listening and for protection from unnecessary

strain.

W. S. TUCKER.

THE SPARROWHAWK

THE NEST AND ITS CONSTRUCTION

ALL the details of the life of the sparrowhawk are very difficult to work out with any real completeness. The building of the nest is no exception, although at first it seems a comparatively simple matter. In the winter months these birds sometimes build flimsy platforms of considerable area, flat, and of no thickness of material. These platforms are usually built of sticks of uniform thickness. They are not intended for nests, and I suppose that very rarely indeed are they added to later on in the spring, and converted into true nests.

Building proper commences early in February, possibly even in January, though such a thing has not come under my observation. I have noted nests completed, except for the last linings, on February 12 and February 17, and others later in the month. The lining of small twigs in the well of the nest is added about a week before laying commences. As a rule the last lining, bits of bark and touchwood and perhaps an odd dried leaf or so, is added during the laying period and the first few days of incubation. Twigs of all sizes are brought to the nest at odd times during incubation and also during the nestling period up to a few days before the nestlings leave the nest to perch about the tree.

My observations lead me to conclude that the male bird of the pair has at least an equal share in the selection of the site for the nest. This conclusion is strengthened by the fact that this is the case with many other birds. It is a real joy to watch a pair of pied wagtails or spotted flycatchers selecting a site for the nest. The male takes a perfunctory interest in any position that the hen suggests and seems to indicate, 'Well, it's your house, but I don't think much of that one.' On the other hand, if the male suggests a place the female gives it a very thorough inspection, and turns herself about in it and tries it to sit in from various angles before rejecting it. An observer can tell immediately the site is selected and go away satisfied that he knows where the nest will be.

Both birds help in building the nest, but the male works much less than the female. He prefers to go off and hunt, and will

sometimes bring his catch and offer it to his mate. At other times he will perch near the nest, and preen himself, occasionally coming up to the nest as if to inspect operations.

My friend Mr. H. S. Davenport, a great bird observer, told me that he had seen both birds building and that they brought sticks in turn, picking them up from the ground. This must have been at the start of the nest, while coarser and heavier material was being utilised. In many nests the first sticks appear as though they might have been got in this way. The later twigs are invariably broken off the trees, and on careful examination the clean new break can be seen. Personally I should say that it was unusual for the birds to get material from the ground or even to pick up material which they have dropped. I think the heavier material has been found lodged in the trees. Since, however, rooks and jackdaws will both break off twigs and fetch them from the ground, there is no reason that hawks should not do so as well.

Although, in my experience, at least 80 per cent. of the hawks build nests for themselves, the remainder use an old nest for a foundation. Often an old hawk's nest is renovated; sometimes an old nest of jay, pigeon, crow or magpie is used, sometimes a squirrel's drey. There is one small wood near Felsted School that has at least ten old hawks' nests in various stages of dilapidation in it, but each year a new nest has been constructed; when this has been looted, the second laying has been usually in an old nest renovated for the purpose. As in the last few years the female occupying that wood has changed three times, the statement is confirmed that the sparrowhawk builds its own nest. Of the old nests used by the sparrowhawk that of the magpie is most remarkable. In the few cases that have come under my observation the magpies' nests were in the very top of lofty oaks in the middle of woods. Very little alteration had been made in the nests, which were in each case some years old and had lost the thorny top protection.

The hawk is essentially a builder in woods, but there are occasional exceptions even to that. I found a nest, on a woodpigeon's nest as foundation, in a large quick fence surrounding the grounds at Leez Priory. In 1922 there was one on Felsted Bury Farm in a large solitary oak in the middle of a meadow. Now and then I have seen one in a hedgerow tree. The usual height above the ground of a hawk's nest is between twenty and thirty-five feet. The lowest nest I have ever seen was on an alder stump on the island in Shrawardine Castle pool. This was less than three feet from the ground, and a brood was reared in it. It would have been a perfect godsend for a bird photographer. The next year the nest was in a very slender alder, a few yards away, and not ten