The earth, being 92,000,000 miles distant from the sun, is 184,000,000 miles farther from Jupiter when at _B_ than it is when at _A_. (See Fig. 180.) It is found by observation that sixteen minutes more are required for the light waves from a reappearing satellite to reach us at _B_ than when we are at _A_. Hence eight minutes would be required for light waves to travel the distance from the sun to the earth. Although light travels at the inconceivable velocity of 186,000 miles per second, the nearest star is so far distant that it takes light three and a half years to come from it to us. The North star requires forty-two years to send its light to us, and Arcturus is so far away that waves of light sent out from it one hundred and sixty years ago are only just reaching us now, and if it should cease to send forth light now men would continue to see it for five generations yet to come. A third kind of wave which goes forth in the ether from the spark gap of our coil is a heat wave. This affects neither our eyes nor our ears, but I will undertake to make you conscious of it by another method. [Illustration: Fig. 181] Before a mixture of gasolene vapour and air can be ignited its temperature must be raised to about 2000 degrees Fahrenheit. I will show that heat waves pass out from this spark gap by placing my watch crystal filled with gasolene underneath the knobs of the spark coil, (Fig. 181). When now I close the electric circuit at the battery the mixture of gasolene vapour and air just above the watch crystal is ignited. If I increase the distance between the knobs you still hear the crackle of the sound waves and see the light waves, but the mixture of gasolene vapour and air does not ignite, because there are not heat waves enough. The automobilist expresses this fact by saying a "fat" spark or a "warm" spark is needed. A battery which has ceased to give a sufficiently hot spark to explode the mixture of gasolene and air in the cylinder of a gasolene engine may serve all other purposes quite as well as ever. It may ring bells almost as long as it ever would. I proved that the temperature for igniting a mixture of gasolene vapour and air was nearly as high as melting iron, by heating an iron rod to a dull red heat and bringing it to the watch crystal containing gasolene. It did not take fire. I showed that it could not be ignited by a lighted cigar, nor even by a glowing coal taken from the fire. It was necessary to heat the iron rod to a very bright red heat--nearly white heat, or nearly to its melting point, before it would ignite the mixture.