shift; he had a mate he has worked with
ever since my father was killed, so I work the other shift with Harvey."

"Now let us look at the plans of the pit," Mr. Hardinge said.

The two inspectors bent over the table and examined the plans, asking a
question of Mr. Brook now and then. Jack had turned to leave when his
employer ceased to speak to him, but Mr. Brook made a motion to him to
stay. "What is the size of your furnace, Mr. Brook?" asked Mr. Hardinge.

"It's an eight-foot furnace," Mr. Brook replied.

"Do you know how many thousand cubic feet of air a minute you pass?"

Mr. Brook shook his head: he left the management of the mine entirely in
the hands of his manager.

Mr. Hardinge had happened to look at Jack as he spoke; and the latter,
thinking the question was addressed to him, answered:

"About eight thousand feet a minute, sir."

"How do you know?" Mr. Hardinge asked.

"By taking the velocity of the air, sir, and the area of the downcast
shaft."

"How would you measure the velocity, theoretically?" Mr. Hardinge asked,
curious to see how much the young collier knew.

"I should require to know the temperature of the shafts respectively,
and the height of the upcast shaft."

"How could you do it then?"

"The formula, sir, is M = h(t'-t)/480+x, h being the height of the
upcast, t' its temperature, t the temperature of the exterior air, and
x = t'-32 degrees."

"You are a strange young fellow," Mr. Hardinge said. "May I ask you a
question or two?"

"Certainly, sir."

"Could you work out the cube-root of say 999,888,777?"

Jack closed his eyes for a minute and then gave the correct answer to
five places of decimals.

The three gentlemen gave an exclamation of surprise.

"How on earth did you do that?" Mr. Hardinge exclaimed. "It would take
me ten minutes to work it out on paper."

"I accustomed myself to calculate while I was in the dark, or working,"
Jack said quietly.

"Why, you would rival Bidder himself," Mr. Hardinge said; "and how far
have you worked up in figures?"

"I did the differential calculus, sir, and then Mr. Merton said that I
had better stick to the mechanical application of mathematics instead of
going on any farther; that was two years ago."

The surprise of the three gentlemen at this simple avowal from a young
pitman was unbounded.

Then Mr. Hardinge said:

"We must talk of this again later on. Now let us go down the pit; this
young man will do excellently wel