"Wright, I've often wondered why it is that oxygen, which combines with almost anything, should be found free in nature. Why is it?"
"I don't know, I'm sure. At that I suppose one reason is that there is so much of it. Just a very small fraction less than half of the Earth's surface layers is oxygen. It forms over forty-nine per cent of it to a depth of ten miles at least. It is the second most active element on Earth—in the universe for that matter, and of the active elements there is only one with which it can't combine, namely, fluorine. Of course it can't combine with the inert gases, so I say the active elements. I suppose it is left free principally because there was nothing else to do. Apparently there weren't enough partners to go around. At that it did a mighty good job of it! Forty-seven per cent. of the solid crust is oxygen, 85% of the water is oxygen, and 20% of the air is free oxygen. Well, let's not look so favorable a gift horse in the mouth. If it hadn't been left free, where would we be?"
The discussion soon died down and the men retired for the night, each wondering what it was that had called Waterson away so suddenly, and each determined to be on hand when he returned in the morning.
The coming of the light of dawn had, perforce, put an end to the activities at Mt. Wilson, so it was shortly after sunrise that the two men heard the hangar doors open. And it was very shortly after sunrise that they had dressed and gone down to greet Waterson. The worried look on his face told a great deal, for both men knew him well, and when Waterson looked worried there was something of tremendous import under way.
"Hello. Had a good night Dave? I have something that is going to interest you—and two and a half billion other human beings. They have discovered something at the Mt. Wilson observatory that is going to change our plans quite a bit. We had intended going to other planets to visit the inhabitants, but we won't have to go. They are coming to us; furthermore, twenty ships are coming, and I have an idea they are good sized ships. But Wright, I think you had better start breakfast. We can discuss it at the table. I'm going to wash, and if you will help Wright, Dave, I think we will be at work pretty soon." Waterson left the room, and the two men looked at his retreating figure with astonishment and wonder. An announcement that our planet was to be invaded from space is a bit hard to take in all at once, and particularly when it is given in the matter of fact way that Waterson had presented it, for he had known it now for over ten hours, and had been working on it during all that time.
At the table the explanation was resumed.
"The ships were first sighted in the big telescope when they turned it toward Mars last night. You remember that Mars is at its closest now, and they are taking a good many pictures of it. When they saw these spots of light on the disc of Mars they were at once excited and started immediate spectroscopic and radiometric observations. The fact that they showed against the disc of Mars meant that they were nearer than the planet, and by measuring the amount of energy coming from them they tried to calculate their size. The results at once proved that they could not be light because of reflection, for the energy that they emitted would require a surface of visible dimensions, and these were points. Their temperature was too low to be incandescent, so they were violating all the laws of astro-physics. By this time they had shifted sufficiently to make some estimate of their distance, shifted because of the movement of the Earth in its orbit, Dave, and so they were covering a different spot on the disc of Mars. Allowing that they were going in a straight line, they were some ten and a half million miles away. The spectroscope showed by displacement of one of the spectral lines that they were coming toward us at about 100 miles a second. The line of their flight was such that they would intercept the Earth in its orbit in about thirty hours. That means that we have about twenty to work in.
"It doesn't take any alarmist to guess that this means trouble. They would not be coming in twenty ships if they were coming on a peaceful mission. Also considering that they come in only twenty ships it shows that they have considerable confidence in those twenty. Since they are coming here without first sending a scouting party of one or two ships, I suspect that they already know that the conditions of Earth are suitable to them. To determine our conditions would require exceedingly powerful telescopes, but they are helped by the thin air of their planet. I believe that they can actually see our machines and weapons, and that they know just about what we have. I think that they are counting on cleaning up the world very easily—as indeed they would but for one factor, for they have atomic energy. Wright, do you remember that we decided to use electronic rockets to drive the car, once we discovered atomic energy? And that having discovered material energy, we naturally decided not to? Well, they have electronic rockets. This makes me feel sure that that means that they have atomic energy, but have no material energy."
"Fine Steve. Your reasoning is most admirable—but will you please translate 'electronic rocket' and a few of those other terms into English? And otherwise make yourself clear to the layman?"