de of
the car and made his way along, holding the top of the block, while the
dust rolled about him and he thought he would be jolted off. Indeed,
there was only an inch-wide ledge of smooth iron to support his foot,
which slipped once or twice; but he reached the brake-gear and screwed it
down. Then, crawling back, he hooked on the spare coupling and returned,
breathless and shaky, to his engine. A minute or two later he brought it
to a stop and had got down upon the line when somebody called him.

Looking round, he saw Fuller standing near, and knew him as the man who
had given him the dollar in the American town. He had heard that his
employer had come out to see what progress was being made, but had not
yet encountered him. He did not notice Ida, who was sitting in the shadow
of the rock.

"You were smart," said Fuller. "There'd have been an ugly smash if the
blocks had got away down the grade. But why didn't you stick to the
throttle and send your fireman?"

"I don't think he understood what he ought to do, and there was no time
to explain."

Fuller nodded. "So you did it yourself! But why didn't you push the car?
You could have held her up better then."

"I couldn't get behind it. The loop-track down at the switches has caved
in."

"I see. But it's a stiff grade and you didn't seem to be hustling your
engine much."

"The boiler was priming and I was afraid of the cylinders."

"Just so. You pumped up the water pretty high?"

"No; it was at the usual working level," said Dick, who paused and
resumed thoughtfully: "I can't account for the thing. Why does a boiler
prime?"

There are one or two obvious reasons for a boiler's priming; that is to
say, throwing water as well as steam into the engine, but this sometimes
happens when no cause can be assigned, and Fuller saw that Dick did not
expect an answer to his question. It was rather an exclamation, prompted
by his failure to solve a fascinating problem, and as such indicated that
his interest in his task was not confined to the earning of a living.
Fuller recognized the mind of the engineer.

"Well," he replied, "there's a good deal we don't know yet about the
action of fluids under pressure. But do you find the grade awkward when
she's steaming properly?"

"I can get up. Still, I think it will soon cost you as much in extra fuel
as it would to relay this bit of line. Two hundred cubic yards cut out at
the bend would make things much easier."

"Two h