In the pursuit of the U-boat the airplane was also extremely effective. These were sent out to patrol large districts near the Allied coast, and also, in some cases, from ships themselves. It is possible in certain weather conditions for the observer on an airplane to detect a submarine even when it is submerged and the airplane can not only attack the submarine by dropping depth bombs, but it can signal at once the location of the enemy to the hurrying destroyers. Indeed, as the submarine warfare proceeded the main difficulty of the Allies was to locate the submarines. Many ingenious devices were used for this purpose, and many of the English vessels had listening attachments under water which were intended to make it possible to hear a submarine as it moved. These, however, do not seem to have been very effective. The submarine itself seems at times to have been fitted out in a similar way and to have thus been able to hear the sound of an approaching ship. Many thrilling reports of naval actions against German submarines were given out officially by the British admiralty from time to time. In most of these cases the submarine was both rammed and attacked by depth bombs. In nearly all of them the only proof of success was the oil and air bubbles which came to the surface. One interesting encounter was that in which a British submarine sighted a German U-boat, while both were on the surface. The British submarine dived and later was able to pick up the enemy through the periscope and discharge a torpedo in such a way as to destroy the German vessel. When the British submarine arose it found a patch of oil in which Germans were swimming. Ordinarily, however, a submarine was of little service in a fight against another for the radius of sight from a periscope is so short that it is practically blind so far as another periscope is concerned. This blindness of the submarine was taken advantage of by the Allies in every possible way. Merchant ships were camouflaged, that is painted in such a way that they could not be easily distinguished at a distance. In the great convoys ships were often hidden by great masses of smoke to prevent a submarine from finding an easy mark. At night all lights were put out or else so shaded as not to be seen by the enemy. The result of these methods was the gradual destruction of the U-boat menace. In the summer of 1918, while occasionally some ship was lost, the production of new ships was much greater than those that were sunk. During the month of June it was announced that the completion of new tonnage by the Allies had outstripped the losses by thousands of tons. During this period the United States had attained its full stride in building ships, airplanes and ordnance.