The metalsmith located a landmark on the inner slope of the valley, set up his heater, and commenced cutting a deep horizontal groove or step in the rock. He kept it always at the same level by following a chalk mark drawn along the rock wall. Libby enquired how the job had been surveyed so quickly.
“Easy,” he was answered, “two of the quartermasters went ahead with a transit, leveled it just fifty feet above the valley floor, and clamped a searchlight to it. Then one of ’em ran like hell around the rim, making chalk marks at the height at which the beam struck.”
“Is this roof going to be just fifty feet high?”
“No, it will average maybe a hundred. It bellies up in the middle from the air pressure.”
“Earth normal?”
“Half Earth normal.”
Libby concentrated for an instant, then looked puzzled. “But look — This valley is a thousand feet long and better than five hundred wide. At half of fifteen pounds per square inch, and allowing for the arch of the roof, that’s a load of one and an eighth billion pounds. What fabric can take that kind of a load?”
“Cobwebs.”
“Cobwebs?”
“Yeah, cobwebs. Strongest stuff in the world, stronger than the best steel. Synthetic spider silk, This gauge we’re using for the roof has a tensile strength of four thousand pounds a running inch.”
Libby hesitated a second, then replied, “I see. With a rim about eighteen hundred thousand inches around, the maximum pull at the point of anchoring would be about six hundred and twenty-five pounds per inch. Plenty safe margin.”
The metalsmith leaned on his tool and nodded. “Something like that. You’re pretty quick at arithmetic, aren’t you, bud?”
Libby looked startled. “I just like to get things straight.”
They worked rapidly around the slope, cutting a clean smooth groove to which the ‘cobweb’ could be anchored and sealed. The white-hot lava spewed out of the discharge vent and ran slowly down the hillside. A brown vapor boiled off the surface of the molten rock, arose a few feet and sublimed almost at once in the vacuum to white powder which settled to the ground. The metalsmith pointed to the powder.
“That stuff ‘ud cause silicosis if we let it stay there, and breathed it later.”
“What do you do about it?”
“Just clean it out with the blowers of the air conditioning plant”
Libby took this opening to ask another question. “Mister — ?”
“Johnson’s my name. No mister necessary.”
“Well, Johnson, where do we get the air for this whole valley, not to mention the tunnels? I figure we must need twenty-five million cubic feet or more. Do we manufacture it?”
“Naw, that’s too much trouble. We brought it with us.”
“On the transport?”
“Uh huh, at fifty atmospheres.”
Libby considered this. “I see — that way it would go into a space eighty feet on a side.”
“Matter of fact it’s in three specially constructed holds — giant air bottles. This transport carried air to Ganymede. I was in her then — a recruit, but in the air gang even then.”
In three weeks the permanent camp was ready for occupancy and the transport cleared of its cargo. The storerooms bulged with tools and supplies. Captain Doyle had moved his administrative offices underground, signed over his command to his first officer, and given him permission to proceed on ‘duty assigned’ — in this case; return to Terra with a skeleton crew.
Libby watched them take off from a vantage point on the hillside. An overpowering homesickness took possession of him. Would he ever go home? He honestly believed at the time that he would swap the rest of his life for thirty minutes each with his mother and with Betty.
He started down the hill toward the tunnel lock. At least the transport carried letters to them, and with any luck the chaplain would be by soon with letters from Earth. But tomorrow and the days after that would be no fun. He had enjoyed being in the air gang, but tomorrow he went back to his squad. He did not relish that — the boys in his squad were all right, he guessed, but he just could not seem to fit in.