About eighteen years ago I had a short telephone-line between my house and that of one of my neighbors. This line was equipped with what was known in those days as magneto-transmitters, such as we have described in a previous chapter on the subject of telephony. When a line is equipped in this way no batteries are needed, as the voice generates the current, on the principle employed in the dynamo-electric machine. Often on summer evenings, when the sky appears to be cloudless, we can see faint flashes of lightning on the horizon, an appearance which is commonly called "heat-lightning." As a matter of fact, I do not suppose there is any such thing as heat-lightning, but what we see is the effect of very distant storm-clouds. Often at such times I have held the telephone receiver to my ear and could hear simultaneously with each flash a slight sound in the telephone. This effect could be produced in the earth by a simple discharge between two or more clouds, which would distribute the electrical discharge over a greater area. And because my line had connection with the earth it could have been disturbed electrically by conduction instead of induction; or it may have been the effect of ether-waves set up by the lightning discharges. There is no doubt in my mind but that both of these effects (ether-waves and conduction through earth) may be felt when a discharge takes place between a cloud and the earth. If we could, by operating an ordinary telegraphic key, cause the lightning to discharge from cloud to earth, and some one was listening at a telephone in a circuit that was grounded at both ends 100 miles or more distant from the cloud, the man who controlled the discharges by the key could transmit the Morse code through the earth to the man who was listening at the telephone. Thousands of people might be listening at telephones in every direction from the transmitting-station, and they would all get the same message. If the receiving-station is near to the point where there is a heavy discharge from the clouds to the earth the earth-current is very strong--flowing out in every direction. For some years I had an underground line between my house and laboratory, and no part of the line between the two stations was above ground. Many and many times during the prevalence of a thunder-storm have the telephone-bells been made to ring at both ends of the line by a discharge from the cloud to the earth, and in some cases the discharge was several miles away. The wires could not have been affected so powerfully in any other way than through the earth. It will be seen by the foregoing statements that it is possible to transmit messages through the earth for long distances, but the difficulty in the way of its becoming a general system is twofold. First, we cannot always have a thunder-cloud at hand from which to transmit our signals, and, secondly, the signals would be received alike at every station simultaneously. CHAPTER XXII.