Aw come on Dave you know that there are people in your readership who are just as capable as Google in giving an answer.
Barrage balloons were used primarily to protect fixed installations or cities. The idea was to force aircraft - specifically dive bombers - to drop their bombs from higher altitudes thereby reducing accuracy and or to force a low flying plane to change course repeatedly rather than have a straight bombing run. The cables with which a barrage balloon was tethered would be dangerous to planes - they could rip a wing off for example. The British deployed more than 400 barrage balloons over London in 1940 (during the Blitz) and by 1944 there were over 3000 barrage balloons in England. The Balloon Barrage destroyed over 200 V-1 flying bombs in 1944.
