The oxygen became rapidly exhausted and the lights grew dim.

Even the valve supplying fresh air for the nostrils of the occupants of the boat would not work and the situation grew more desperate with the flight of every second.

As the atmosphere became oppressively heavy, Boyton wanted to knock the valve off with a hammer; but the engineer showed him if that were done, they would be drowned.
It began to dawn on the minds of the three men that they were doomed.
They sat and looked into one another's pale faces.

Paul consulted his watch and estimated that twelve more minutes must elapse before those above would haul up.

He felt that it would be impossible for them to last so long for already they were beginning to gasp for lack of air.
They became weak; but again tried the valves to no purpose.