<P> A barrage balloon is a large kite balloon used to defend against aircraft attack by raising aloft cables which pose a collision risk, making the attacker's approach more difficult . The design of the kite balloon, having a shape and cable bridling which stabilise the balloon and reduce drag, meant that it could be operated in higher wind conditions than a spherical balloon . Some examples carried small explosive charges that would be pulled up against the aircraft to ensure its destruction . Barrage balloons are not practical against very high - flying aircraft, due to the weight of the long cable required . </P> <P> France, Germany, Italy and the United Kingdom used barrage balloons in the First World War . Sometimes, especially around London, several balloons were used to lift a length of "barrage net", in which a steel cable was strung between the balloons and many more cables hung from it . These nets could be raised to an altitude comparable to the operational ceiling (15,000 ft / 4,500 m) of the bombers of that time period . By 1918 the barrage defences around London stretched for 50 miles (80 km), and captured German pilots expressed great fear of them . </P> <P> In 1938 the British Balloon Command was established to protect cities and key targets such as industrial areas, ports and harbours . Balloons were intended to defend against dive bombers flying at heights up to 5,000 feet (1,500 m), forcing them to fly higher and into the range of concentrated anti-aircraft fire--anti-aircraft guns could not traverse fast enough to attack aircraft flying at low altitude and high speed . By the middle of 1940 there were 1,400 balloons, a third of them over the London area . </P> <P> While dive - bombing was a devastatingly effective tactic against undefended targets, such as Guernica and Rotterdam, dive - bombers were very vulnerable to attack by fighter aircraft while performing a dive, and their use in this role by Germany against the UK with its effective Royal Air Force was rapidly discontinued . Balloons proved to be of little use against the German high - level bombers with which the dive - bombers were replaced, but continued to be manufactured nonetheless, until there were almost 3,000 in 1944 . They proved to be mildly effective against the V - 1 flying bomb, which usually flew at 2,000 feet (600 m) or lower but had wire - cutters on its wings to counter balloons. 231 V - 1s are officially claimed to have been destroyed by balloons . </P>

What were the blimps used for in ww2 in london