Before the mine goes over, a windlass inside the plummet-sinker is revolved by hand until the length of cable between the plummet and the anchor-chamber has been reeled off equivalent to the depth below the surface at which the explosive mine is to float. Then the entire apparatus is hove overboard. The plummet and anchor-chamber sink, while the spherical mine proper is kept on the surface for the moment by means of a buoyant air-chamber within. A windlass in the anchor-chamber now pays out the cable between it and the mine as the anchor-chamber sinks. On the plummet touching bottom, the tension in the cable between it and the anchor-chamber is lessened, and the windlass mentioned stops. The anchor-chamber thereupon sinks to the bottom, dragging down the spherical mine until that is at the selected depth ready for its deadly work. CHAPTER XVII AERO-MILITARY OPERATIONS _Aerial Attacks on Cities--Some of the Achievements of the Airmen in the Great War--Deeds of Heroism and Daring--Zeppelins in Action--Their Construction and Operation._ During the first ten weeks of the war German airmen flew over Paris several times and dropped bombs that did some damage. Aeroplanes, not Zeppelins, were used in these attempts to terrorize the capital and other cities of France. The early visits of Zeppelin airships to Antwerp have been described in a previous chapter. These were continued up to the time of the fall of Antwerp. While comparatively few lives were lost through the explosion of the bombs dropped, the recurring attacks served to keep the inhabitants, if not the Belgian troops, in a state of constant excitement and fear. When the city fell into German hands, a similar condition arose in England, where it was feared that Antwerp might be made the base for German airship attacks on London and other cities of Great Britain; and all possible precautions were taken against such attacks. The members of the Royal Flying Corps were kept constantly on the alert; powerful searchlights swept the sky over London and the English coast every night and artillery was kept in readiness to repel an aerial invasion. Such was the condition in the third week of October. BRITISH ATTACK ON DUSSELDORF