Twenty hours after the fleet had turned back, the radio barrage was again lowered over the System. It was ten hours later that the Sirians reached Venus.
While the radio barrage had been lifted, Waterson had had an idea that there should be some protection for the planet. It did not seem that the planet should be completely stripped of its defenses, and he had suggested that at every city great Dis ray machines of the sixty-mile range type be set up. His suggestion was followed, and at every city on Venus the great machines were installed. There were many of them now, for during the hundred hours the main fleet was in flight the new machines had been put on a quantity production basis. But all the ships that were equipped with them, were sent to the defense of the unattacked Earth! And it was those machines that prevented the landing of the Sirians. They came to the night side of the planet, of course, coming from Mars. It would be thirty hours before they would be expected on Earth—thirty hours before the main fleet would reach the planet—and then there would be the 160,000,000-mile trip to Venus if they were to get there in time to rescue the planet.
But the Sirians could not approach within beaming distance of the cities, and all those that did try to do so, were brought down as a cloud of powdery dust. It was Waterson's caution that saved the billions of people on Venus.
But were they to be saved? The Sirians decided they must destroy the works and the people on Venus, so they made one desperate effort. They had at least sixty hours to work in, and now they had a plan that would require time. They retired some hundred miles from the planet, then the entire fleet, torpedo-ships, cargo boats, and the entire body guard of the Sphere lined up, and then switched on powerful attractor beams. Immediately, the combined effect of over three million of these emanations took hold on the planet, and great tides began to rise in its mighty oceans. Many lives were lost in the seaside towns, when the tremendous waves rushed in over the land. But astronomers on the planet and most of the System's scientists were there to watch the Sirians on Mars through their great telescopes. And these astronomers saw what the Sirians intended, and saw that they were well on their way to fulfilling their aim.
A planet is balanced in its orbit about its parent sun with the delicacy of a diamond on a jeweler's scales. But, like the diamond, if it be displaced by some force, it reaches a new state of equilibrium. Thus, if the diamond is further lowered in the scale by adding a small weight, it soon reaches a new point of equilibrium. No conceivable force, therefore, could be great enough to displace the planet in its orbit more than a few million miles by pulling it either in toward the sun, or out from it, and as soon as that force was released, it would spring back to its original position as the diamond would regain its balance on removing the disturbing weight. For the sun pulls on a planet with a titanic force; it draws it in with the apparent force gravity, and another similar, but opposite force, centrifugal force of its revolution in its orbit, is constantly tending to throw it into the depths of space. These are the two forces that are always balanced. Suppose the planet is drawn nearer the sun; it revolves in a smaller orbit—and it revolves in that smaller orbit with a higher speed—for it has fallen in toward the sun; it has gained speed as any falling body would. It has gained speed in the direction of the sun, but this has operated to increase its rotational speed. Thus it has gained a greater centrifugal force—you can see the effect with a bit of chalk on the end of a string. The smaller the circle it swings in, the greater the tendency to fly outward. But as long as we continue the force that was added to draw it in, it will remain in equilibrium. Remove this extra force and at once the planet will fall away from the sun, losing speed as it does so, till it has reached a point where it is once more in equilibrium with the force drawing it inward.
Now reverse the problem. Let us draw it away from the sun. Now the orbit is longer, and it has lost speed in moving from the sun. It cannot stay here, it is not in equilibrium, unless the force that drew it out is maintained. To free the planet from the sun, one would have to lift hundreds of quintillions of tons of rock through billions of miles, against the terrific gravity of the sun. It is too much.
Thus we see that as long as the planet revolves in its orbit, it will never fall, and to pull it away from the sun is impossible as long as it revolves in its orbit. But if it slows down in its flight about the sun it at once has less centrifugal force. It automatically falls toward the sun until it has gained velocity enough to establish a new orbit of equilibrium. If this energy, too, is withdrawn; if it is made to stand still in its orbit; it will fall straight to the sun. It is the only way such a thing might be done. And it would take the energies of matter, and strain that to the utmost, to accomplish it.
This was the plan of the Sirians. Three million ships were dragging like a Titanic brake on the planet as it wheeled in its orbit, and slowly, steadily it was falling into the blazing furnace of the sun. Their ships were not designed for this task, but they could do it in the sixty hours at their disposal. In a short time it would be falling directly toward the sun, but it would take many hours for the seventy-million-mile fall. Even if it were stopped before it reached the sun, any place within twenty million miles would be unbearable.
It was the distressed planet itself that warned the people on Earth and the men of the fleet that the Sirians would never reach Earth, for the radio was still dead. But the fleet turned for Venus at once. They were far to one side of the path to Venus, and they would have to turn, but it would take them thirty, instead of sixty hours to reach Venus. And the other fleet was coming from Earth. They were not quite a million strong, but those machines that had been produced on Venus would come also, and that would bring the total numbers up to over a million, and with the main fleet the number would be well over two million. There were also three hundred of the long range Dis ray ships now, for many more had been produced and Venus would supply an equal number.