One evening we were seated about the dinner table when Harold asked me how electricity could travel without wires. I replied, "It travels as light does. But I am very much puzzled to know why it ever follows a wire when light does not." This did not settle the question and left us both unsatisfied, so I told him to invite two or three of his best friends in to-morrow evening, and I would perform some experiments for them that would at least help them to think further upon this subject. When the evening came I showed the boys an automobile spark coil to which I had attached two knobs, _a_ and _b_ (Fig. 179), and with which I had connected two dry battery cells. When I touch the wire _c_ to the binding post _d_ a spark passes between the knobs _a_ and _b_. When this spark occurs at least four kinds of waves pass out in all directions from the spark gap between the knobs. [Illustration: Fig. 179] First, sound waves go through the air. Our ears detect these. If the air is removed from around the apparatus no sound wave can go forth. A careful examination of the internal ear shows us that it is constructed so as to respond to such air waves. Second, light waves go forth. These affect our eyes. We are blind to the first kind of waves and deaf to the second. The light waves travel without air--somewhat better without air than with air. A microscopic examination of the eye indicates that it is constructed so as to respond to waves. We believe there are waves in the ether which fills all space. Sound waves travel in air at the rate of one mile in five seconds. We had this nicely illustrated at the sea shore one summer. The steamer touched each morning at a wharf which we could plainly see two miles distant. We could see the steam arise when she blew the warning whistle, and with our watches we found that it always required ten seconds for the sound to reach us after we saw the steam of the whistle. This at least showed us that it takes five seconds longer for sound waves to travel a mile than it does for light waves to travel the same distance. For light had to travel the same distance before we could see the steam arise from the whistle. Although the time it takes for light to travel a mile is inconceivably small, we have a simple method of finding out that it requires eight minutes for light waves to come to us from the sun. The satellites of the planet Jupiter, in revolving about that body, disappear and reappear at regular intervals, acting as flash lights to mark time. [Illustration: Fig. 180]