"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

"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.