[Illustration: Fig. 193.—Little Waves spread out from the Spot.] It exists in a vacuum and in solid rocks. Since the ether does not make itself apparent to any of our physical senses, some of these statements may seem contradictory. Its definite existence cannot be proved except by reasoning, but by accepting and imagining its reality, it is possible to understand and explain many scientific puzzles. A good instance is offered by the sun. Light and heat can be shown to consist of extremely rapid vibrations. That fact can be proved. The sun is over 90,000,000 miles away from our earth and yet light and heat come streaming down to us through a space that is devoid even of air. Something must exist as a medium to transmit these vibrations; it is the ether. Let us consider again the pool of water. The waves or ripples caused by throwing in the stone are vibrations of the water. The distance between two adjacent ripples is called the _wave length_. The distances between two vibrations of light can also be _measured_. They are so small, however, that they may be spoken of only in _thousandths_ of an inch. The waves created in the ether by wireless telegraph apparatus are the same as those of light except that their length usually varies from 75 to 9,000 _feet_ instead of a fraction of a thousandth of an inch. [Illustration: Fig. 194.—A Simple Transmitter.] *A Simple Transmitter* is illustrated in Figure 194. A telegraph key is connected in series with a set of cells and the _primary_ of an induction coil, which, it will be remembered, is simply a coil consisting of a few turns of wire. This induces a high voltage in a second coil consisting of a larger number of turns and called the _secondary_. The terminals of the secondary are led to a spark-gap—an arrangement composed of two polished brass balls, separated by a small air-gap. One of the balls, in turn, is connected to a metal plate buried in the earth, and the other to a network of wires suspended high in the air and insulated from all surrounding objects. When the key at the transmitter is pressed, the battery current flows through the primary of the induction coil and generates in the secondary a current of very high voltage, 20,000 volts or more, which is able to jump an air-gap in the shape of a spark at the secondary terminals. The latter are connected to the earth and aerial, as explained above. The high potential currents are therefore enabled to charge the aerial. The charge in the aerial exerts a great tendency to pass into the ground, but is prevented from doing so by the small air-gap between the spark-balls until the charge becomes so great that the air-gap is punctured and the charge passes across and flows down into the ground. The passage of the charge is made evident by the spark between the two spark-balls.