A defect of this type was its inability to eliminate certain sounds which had nothing to do with aircraft, such as those from petrol tractors, motor bicycles, etc., on the ground in the neighbourhood. The Germans in Belgium had to give up using their instrument at a searchlight near Bruges, because of the noise made by the frogs in the dykes all round it.
Another pattern of sound locator has been constructed in cliffs on the shore by cutting a concave surface in the face of the cliff, and providing an appliance for collecting the sound waves at their point of maximum concentration, in such a way as to indicate approximately the direction of the source of the sound. This pat- tern was used on the British coasts and warned observers of the approach of machines from Belgium when the latter were as far away as 15 to 20 miles. These locators also, however, were liable to error; and on one occasion a fleet of motor-boats caused an alarm which was only prevented from becoming public by the perspicacity of the local anti-aircraft commander.
The functions of the observer post, which may or may not be equipped with detector apparatus, are of great importance. The duty of the observers is to detect the passage of aircraft and report their movements to the authority controlling the air defences. On these reports depend the warnings to the civil and military authorities within the defended areas. Such duties demand considerable physical strength to bear the severe strain incurred by watching and waiting; and a high degree of refine- ment in hearing and eyesight, owing to the necessity of detecting and identifying aircraft at great distances. The speed of aircraft in the attack is the factor which determines the minimum dis- tances of the posts from the objective, and those distances may involve the disadvantage of great isolation for many posts. The necessity of good and speedy means of communication between such posts and the controlling air defence authority to which they belong is obvious. However excellent an observer's training may be, a report based solely on what he has heard must be open to doubt, should it attempt identification of the class of the air- craft in question.
(e) The aerial obstacle consists of some form of wire impediment hung from balloons, and intended to be such a menace to a flying machine that it will either pass beside the obstacle or, more prob- ably, fly at a higher level than the balloons supporting it.
The Italian authorities claimed extraordinary success with the contrivances used in their defences. The French authorities were not so optimistic over the type adopted by themselves, and there is no proven case of success with the pattern used in Great Britain.
The Germans at Bruges and Zeebrugge flew kites and balloons by wires of a very high tensile strength, and one Handley Page bombing machine with its crew was destroyed at Bruges by these means. The balloons were about 15 ft. in diameter, and were used when wind power was insufficient to raise a kite. The kites were of at least two patterns, but both were of the double box-kite type. The lower ends of the wires were wound on vehicles- provided with gauges, oil baths, and lightning "earths." They were managed by a few small boys, pressed into the German service at the rate of a few francs a night. It was calculated that the wire provided an obstacle up to about 3,000 feet.
In Great Britain balloons moored by wire cables were arranged in lines, and at some distance below the balloons was a bridle con- necting all the cables. From this bridle at equal intervals were sus- pended long thin wires of considerable tensile strength. 1
Any arrangement of obstacles suspended from balloons must be particularly vulnerable, both from the shell fire of the defence and from any machine-gun fire brought to bear by the attack.
Many other forms of obstacles have been suggested from time to time, and perhaps one of the most ingenious was that of an aerial minefield. The inventor proposed to attach a small charge of explo- sive, sufficient to destroy an aeroplane wing, to a revolving vane by a length of fine cord. The charge was fitted with suitable per- cussion firing arrangements. The vane was attached for the purpose of delaying the fall of the explosive through the air. Charge, cord, and vane were neatly packed together so that considerable numbers could be carried in a box provided with a simple release. The pro-
1 A curious incident occurred during an air-raid alarm in London during the war. To the astonishment of the detachments one com- plete series of balloons came down with unexpected suddenness, all being deflated by the rupture of their ripping panels. On examina- tion, it was found that moisture had condensed on the ripping ropes and frozen there, until each cord was about as thick as a man's forearm. The weight had gradually increased on all with remark- able regularity until the ripping point was reached, when each bal- loon in the series was deflated almost simultaneously. There was a heavy mist that night, and the temperature at the ground level was above freezing-point.
cedure proposed was to send up a group of machines loaded with these " mines," on patrol well outside the defences, on any occa- sion when conditions were so favourable that a raid was probable. The " mines " were to be released across a broad belt through which the attack would probably pass, as soon as a signal was made from the ground that it had reached a suitable point on its course. The idea, however, was never carried into effect probably owing to the danger involved to friendly machines, but nevertheless it had possibilities which gave considerable promise, especially for use over the sea.
(/) Bombproof and Splinterproof Protection. Local protection for personnel, animals and stores involves the provision of shelters proof against the bombs themselves and their splinters. A bomb with a stout and heavy point, and provided with a means to keep it revolving so that the point travels first, will, if launched from a great height, penetrate most practicable forms of shelter. A stout double roof of concrete with the sides sloping fairly steeply, and provided with a " sandwich " of some resilient material between the roofs, will probably give protection to what is beneath it, provided that the foundation supporting the roofs is a good one.
Many bombproof shelters made of concrete or big stones cemented together were constructed in France and Belgium by all belligerents, but they were generally of small capacity, and provided for particu- lar detachments whose duties necessitated their remaining in the vicinity at all times. It is not, however, possible to provide such pro- tection universally; in most cases all that can be done is to minimize the danger as far as is practicable, and to accept the fact that a direct hit on, or an explosion very close to, the person or animal will finish the matter as far as they are concerned. A little-known fact is that the open spaces in a big city like London may amount in total area to nearly ten times that on which houses are actually built, the chance that a bomb will fall on a house being therefore far less than is generally recognized.
The heaviest bombs used by both sides in the war made craters about 35 ft. in depth when dropped on ordinary soil and open ground. These bombs were fitted with fuses with a slight delay action. A light bomb with a very sensitive fuse was used by the Germans with deadly effect against men and animals. The crater made by it was practically negligible, all the fragments flying out- wards and upwards. Protection against this type of bomb was afforded by low parapets of sandbags or sods, close to which troops could live; but horses were extremely difficult to protect against these so called " daisy cutters." In the open, protection during a bombing attack will generally be best found by lying down in a depression if one be available.
In houses it is difficult to say which position is the safest; a bomb with a delay-action fuse will probably blow the whole house up from roof to cellar, while one with an instantaneous fuse will probably blow the roof in. On the whole, it would appear that the safest position of all is near the chimney breast in a room on the first floor, and below the level of the window sill. Such a spot may give protection from debris falling from the roof, and from splinters from a bomb bursting in the road outside, but is of course not likely to be of any use if the whole house is blown up.
Torpedo nets arranged in tiers about 10 ft. above each other may provide a certain amount of protection against small bombs fitted with instantaneous percussion fuses, but they are costly and diffi- cult to erect.
Camouflage. Concealment of the ground target may take more than one form. The landscape may be studied from the air, and the vulnerable points treated in such a way with painting and netting and so on, as to assimilate their appearance as far as possible with the surrounding country. Again, attempts may be made to hide an important point with smoke clouds during a raid, but unless the work is very carefully done the smoke may invite attention to the possible objective rather than conceal it. In any case it involves much careful organization, and may in the end prove very expensive. Lights and dummy buildings may be placed in exposed positions so as to form attractive targets for hostile bombing machines, at a safe distance but not too far from the point actually sought by the enemy. Thus a carefully arranged target of green, red, and white lights may successfully simulate and so protect an important railway junction.
Concealment of the principal leading-in marks has frequently been suggested ; but success would only be likely with objects which were of small size, and therefore probably of comparatively small importance. For a big objective such as London, where there are such prominent guides in the nature of rivers, railroads and valleys the expense of concealment would be enormous and the probability of success negligible. Moreover, the developments of wireless teleg-