For a condenser he used two metal plates a little distance apart, the air between forming the insulating material. He set up his apparatus in a large room, and having started the coil he moved about with a nearly complete hoop of wire, the ends of which nearly touched. Working in darkness he found after a while that sometimes he could see little sparks, very small but just visible across the gap between the ends of the bent wire. Those sparks only occurred when the coil was in action, and so he knew that the one was the result of the other's work. By careful painstaking experiment he found that the sparks were unquestionably caused by waves, and that the waves moved with the same speed as light, also that they could be reflected and refracted just on precisely the same principles as those which control light. Moreover, he measured the wave-length. At first sight it seems incredible that anyone could measure the distance apart of waves which travel at such a speed as 186,000 miles per second, but fortunately, by a special application of "interference," it is possible to make the waves stand still and tamely submit to measurement. An example of this can be seen by simply tapping a glass of water, when the ripples being reflected off the sides interfere with each other and become stationary. Stationary waves are half the wave-length of the original waves, and by using this method Hertz was able to make a measurement which at first sight seems beyond the bounds of possibility. Thus Hertz discovered how to make the waves which Clerk-Maxwell had predicted and also how to detect them when made. It was not long before the idea arose of using these waves for signalling to a distance. Many experiments were made but with no very striking success until 1896 when Marconi first came to England. Hertz had noticed that the farther apart he placed the plates of his condenser the farther could he get his tell-tale spark, so Marconi saw that the plates of his condenser, too, must be far apart. He also found that the earth could be used as one of the plates, that in fact there was a great advantage in so using it. So, one plate having to be the earth itself and the other removed as far as possible from it, the tall masts of the wireless antenna came into being. [Illustration: LISTENING FOR THE ENEMY. Special sensitive cylinders are sunk into the ground to which the usual telephonic apparatus is fixed. This enables the sappers to detect any underground operations by the enemy.] When Marconi came to England he was taken under the kindly wing of Sir William Preece, the veteran engineer of the Post Office, and the facilities which Sir William was able to give no doubt helped largely in his subsequent rapid progress. After a few experiments in London he got to work across the Channel, sending messages from the North Foreland Lighthouse to Wimereux on the coast of France, including congratulatory messages between the French authorities and good Queen Victoria.