Flying at one hundred miles per hour, we should not reach the moon for three months. And the moon is our nearest neighbor in space. The sun is four hundred times as far away as the moon, so it would take us a hundred years to reach the sun. And even this far-distant sun seems a near neighbor when we think of the distance from us to the fixed stars. To reach the nearest of the stars it would take us millions of years. I am not going to ask you to imagine yourself living for millions of years, shut up in the cabin of our flying machine. It is quite evident that we must increase the speed of our machine. Suppose we make it go as fast as a ray of light can travel! How fast is that?
The speed of light is so great than any race we set for it on this earth seems no time at all. You know that it takes us several months to travel round this world in a steamer. Of course we could go far faster in a flying machine. But even if we could keep flying all the time at one hundred miles per hour, it would take us ten days.
Suppose now that it were possible to send a ray of light on this same journey around the world. It would have traveled right round before you could count "one". Indeed it could travel five hundred times round the world in one minute. That may help to give you some vague sort of idea of the speed at which our flying machine is to travel.
Even at this unthinkable speed, it would take us about four years to reach the nearest star. Does it really take light several years to travel from a star to us? It does; it takes light hundreds of years to reach us from some of the distant stars. If one of these far-distant stars should disappear suddenly, you and I would know nothing about the change. When we looked up into the sky we should still see the star shining away year after year. Our children and our grandchildren would still believe the star to be where it had been always, for the last rays of light, sent out by the star before it disappeared, would take hundreds of years to reach this world. There is no hope of our ever reaching these very distant stars in our imaginary flying machine; it would be too big a tax on our imagination to live in the cabin of our machine for hundreds of years.
For the present we shall be content with a trip to the moon. But one boy says that he cannot imagine our flying machine forcing its way through the air at the enormous speed at which light travels. Even when traveling by motor car, at the comparative snail-pace of, say, forty miles per hour, we have to see that hats and veils are well fastened on. This boy says that he has been told that a good deal of the additional energy required to drive a train at sixty miles an hour is used up in forcing the train through the ocean of air.
We quite agree with this boy that it would be very difficult to imagine our machine traveling through the air at the speed of light. We shall get over this difficulty by making our machine not exceed one hundred miles per hour so long as we are in the ocean of air. When we get beyond this great blanket of air, we can increase our speed to that of light.
I am very glad that we have come to this arrangement, for I had just been thinking that our journey to the moon would be a very disappointing one, as it would have taken only about one second to get there. Now our journey will be much slower and we shall have time to look about us.
When we emerge from the misty clouds we cannot see the instruments for the moisture on the window. By the time it clears we are about two miles upwards off the earth. We find that the temperature has fallen down below freezing-point. We are rather surprised, as it looks lovely and warm in the sunshine. The moment we shoot above the clouds we are in the sunshine, with the sea of clouds below us. Our thermometer is placed in the shade, so that the direct rays of the sun cannot reach it, as we wish to know the temperature of the surrounding air.
The reason why it is colder here than down on the surface of the earth is that we have climbed out of the thickest part of the great blanket of air which serves to keep you warm. The air is what you might describe as much thicker near the earth, but, to speak more correctly, we should say that the air is much denser at the surface of the earth. Up here it is much flimsier. One girl says that she climbed a high mountain with her father, on a beautiful summer day, and it was so cold on the top of the mountain they could not stay there many minutes.