We covered another two hundred and one miles that day.
On the thirty-fifth day, we covered two hundred and thirty-one miles.
On the thirty-sixth day, we covered two hundred and twenty-four miles.
On the thirty-seventh day, we had covered two hundred and seven miles in the first fourteen and one-half hours.
There wasn't any warning, either in external physical signs or on the tractor's instruments. One minute we were rolling along like a test run at the proving grounds, and the next a four-hundred-foot stream of mercury vapor under pressure was coming out the left side of the tractor.
It lasted only a few seconds. That was all it had to.
I sat and stared for several long minutes, blinking my eyes and trying to see something besides a pure white line. I heard Helene climb slowly down from the cab and go up through the airlock, yet I really didn't hear anything at all.
Finally I got down and turned on my suit light and took a look at the hole.
There wasn't much to see. The hole was no bigger than a small lead pencil, and I probably wouldn't have been able to find it in the dark if it hadn't been surrounded by a slowly contracting area of white-hot metal.
We were lucky. We were incredibly lucky. If that mercury had come out at an angle either one degree higher or lower than it had, we'd have been minus a tread or a chunk of the tractor's body.