“Have you tried to fish it out with a hook and line?”

Johnny’s face brightened.  “No, sir, I never thought of that.”

“All right; you couldn’t do it.  Besides, if you could, it wouldn’t be scientific,” said the school-master.  “Now, go home, take a ten-foot pole, and measure the distance from the eaves to the water in the cistern, then find the diameter of the pipe, and on my way to school to-morrow morning I will tell you the three things necessary for recovering your ball.”

Johnny fairly flew home, got a pole, measured the distance from eaves to water and found it to be twelve feet; measured the pipe and found it to be two inches and one-half.  Then he put away the pole, did his chores, ate a hearty supper, and went to bed.

He was up bright and early next morning, and got quickly through his chores, so that when the school-master stopped, on his way to school, he was ready to see about the ball.

“Good morning, Johnny!  Glad to see you on hand.  How long’s the pipe?”

“Twelve feet, sir.”

“Diameter?”

“Two inches and a half, sir.”

“Ah! 2-1/2 square multiplied by .0034, and that product by twelve feet, which is—­”

“144 inches,” Johnny quickly suggested.

“Will give the contents of the pipe in gallons,” added the schoolmaster.  “You’re quick at figures, tell me the answer.”

Johnny groped among the odds and ends of his jacket pocket for a minute, and then fished out a stubby lead-pencil, much chewed at one end, and picking up a piece of smooth board, ciphered away swiftly and carefully a few moments.

“3.06 is what I make it, sir.”

“Very well; we’ll call that right; that would be a little over a pailful—­say a pailful and a half.  Now get a ladder to go up to the roof with.”

Johnny brought one in a jiffy.

“All right.  Now, the three things necessary to get back your ball are, a pailful and a half of water, a plug, and pluck.”

Johnny looked as if he didn’t quite understand.

“What sort of a plug, sir?” he asked.

“Oh, this will do,” answered the school-master, picking up a pine stick and beginning to whittle away vigorously.  The plug was soon made.  The school-master lifted the plank cover from the cistern put the ladder down, and said to Johnny:  “Have you any pluck?”

“Lots of it,” Johnny told him.

“Well, then, take this plug and stick it into the mouth of the pipe, snug.”

Johnny took the plug, went down the ladder into the cistern till he reached the water, and then began feeling around for the pipe.  By and by he found it, and, inserting the plug in the opening, pushed it down and screwed it firmly in place.

“All right!” he called out, and presently he came up the ladder.

“Now let’s have the water—­in two pails,” the schoolmaster said, and he saw by Johnny’s face that he at last understood how the ball was to be got out.  Johnny ran to the barn, and soon came back with two pails of water and a funnel.