There is another radio invention which we contributed during the war, that proved of utmost service in thwarting German spies and which is going to prove equally valuable in time of peace. Although a war invention, its peacetime service will be to save lives. It is a very simple matter to rig up a wireless-telegraph system that will send messages to a considerable distance, and simpler still to rig up a receiving-set. European governments have always discouraged amateur radiotelegraphy, but in this country restrictions used to be so slight that almost any one could set up and use a radio set, both for receiving and for transmitting. When we entered the war we were glad that amateurs had been encouraged to play with wireless, because we had hundreds of good radio operators ready to work the sets which the army and the navy needed. But this was a disadvantage, too. Many operators were either Germans or pro-Germans and were only too willing to use their radio experience in the interest of our enemies. It was a simple matter to obtain the necessary apparatus, because there was plenty of it to be had everywhere. They could send orders to fellow workers and receive messages from them, or they could listen to dispatches sent out by the government and glean information of great military and naval importance. The apparatus could easily be concealed: a wire hung inside a chimney, a water-pipe, even a brass bedstead could be used for the receiving-aërial. It was highly important that these concealed stations be located, but how were they to be discovered? THE WIRELESS COMPASS