"And this is the way we must use to fight the beings who now cruise far above us! We must mine the air! We must send up thousands of great mines constructed so that they will float up to the surface of earth's atmospheric ocean and remain near that surface where, we know, the ships of these space-beings are coming and going. With thousands of mines floating about some of the invaders will inevitably be destroyed by a collision with one of them." The others were silent for a moment. Then the Secretary of War broke out: "Mine the air!" he repeated incredulously. "But how? How could any mine be constructed to float up in that way, so light as to float on the atmosphere's surface?" By Means of _Steelite_ "There we have recourse again to our analogy," Dr. Howard declared. "If a mine attached to a great hollow globe of air were released at the sea's bottom it would rush up toward the surface and float there. In the same way, a mine attached to a great globe in which is a complete vacuum would rush up through our air-ocean! "We can construct those globes of the strongest and lightest material known--_steelite_. As you all know, _steelite_ is the recently devised metal that has immensely greater strength than steel but the merest fraction of its weight. The tensile strength of a substance, its hardness and other qualities, depend directly upon the arrangement of its molecules. By the recent discoveries of Browning we know that when steelite is synthetically made the arrangement of the molecules is such that it forms an extremely strong and rigid substance. This gives the material unheard-of strength with unheard-of lightness. "A forty-foot globe of _steelite_ can be made, with a charge of explosives of terrific power attached to it that will detonate when the globe is touched. This done, all air is exhausted from the hollow globe by great pumps, until the vacuum inside it is almost complete. The enormous strength of the steelite shell prevents its being crushed by the atmosphere's pressure as an ordinary steel shell would be. "Released, the vacuum globe with its deadly charge will shoot up through the atmosphere with terrific speed. Its lightness for its size will be such that it will not halt until it floats on the very surface of our atmosphere. We will make these globes in countless thousands, each with its charge, and release them. Each globe will have a device that will repel any other, so that they do not detonate each other. We will sow the surface of our atmosphere with these deadly air-mines! Sooner or later the ship or ships cruising to and fro on our atmosphere's surface will strike one and will be destroyed by its detonating charge. In this way, and only in this way, can we fight back against the beings from space who are releasing this horror on us!" "But to make these globes or mines in such numbers will engross all the earth's industrial activities!" the British ambassador exclaimed.