After the station had been located as closely as possible in this way, a motor-truck was sent out in which there was a concealed radio compass. The truck would patrol the region located by the fixed compasses, and with it the position of the concealed station could be determined with perfect accuracy. The building would be raided and its occupants jailed and the radio equipment confiscated. Even receiving-sets were discovered with the portable compass, but to find them was a far more difficult task. For the receiving of messages from distant points without a conspicuous aërial an audion would have to be used and this would set up feeble oscillations which could be picked up under favorable conditions by the portable compass. PILOTING SHIPS INTO PORT And now for the peace-time application of all this. If the compass could be used to find those who tried to hide, why could it not also be used to find those who wished to be found? Every now and then a ship runs upon the rocks because it has lost its bearings in the fog. But there will be no excuse for such accidents now. A number of radio-compass stations have been located around the entrance and approach to New York Harbor. Similar stations have been, or soon will be, established at other ports. As soon as a ship arrives within fifty or a hundred miles of port she is required to call for her bearings. The operator of the control station instructs the ship to send her call letters for thirty seconds, and at the same time notifies each compass station to get a bearing on the ship. This each does, reporting back to the control station. The bearings are plotted on a chart and inside of two minutes from the time the ship gives her call letters, her bearing is flashed to her by radio from the control station. [Illustration: Courtesy of the "Scientific American" FIG. 12. Approaches to New York Harbor showing location of three radio compass stations and how position of a ship sending signals from A may be determined] The chart on which the plotting is done is covered with a sheet of glass. Holes are pierced through the glass at the location of each compass station. See Fig. 12. On the chart, around each station, there is a dial marked off in the 360 degrees of the circle. A thread passes through the chart and the hole in the glass at each station. These threads are attached to weights under the chart. When a compass station reports a bearing, the thread of that station is pulled out and extended across the corresponding degree on the dial. The same is done as each station reports and where the threads cross, the ship must be located. Not only can the direction-finder be used to pilot a ship into a harbor, but it will also serve to prevent collisions at sea, because a ship equipped with a radio compass can tell whether another ship is coming directly toward her. And so as one of the happy outcomes of the dreadful war, we have an apparatus that will rob sea-fogs of their terrors to navigation.