“Yes, and all this land is his, up to Somerset Lane. He is away a great deal and expects me rather to keep an eye on things, but of course I work on the farm too. There is really almost no one else to do it with all the labor crowding into the cities. I try to study by myself at night but—I don’t get very far. There are some places where I think I will stick forever.” “Oh,” exclaimed Betsey, suddenly seeing the explanation of that puzzling page that had fallen into her hands the night David’s papers blew away, “do you find it hard to prove what is the volume of the frustum of a pyramid?” “Do I?” returned David from the bottom of his heart. His freckled face crinkled into a delighted grin as he looked up at her. “Don’t tell me that there is some one else who finds it as mysterious as I do! ‘The frustum of a pyramid is equal to three other pyramids,’ I can get that far and I can even understand the first one, but the second is almost too much for me, and the third is quite impossible.” “And I,” returned Betsey gravely, “if I can once get the first one, I can go on to the end. It is just at the beginning that I always come to grief.” “Perhaps you could help me and I could help you,” David suggested excitedly. “I would sleep easier at night if I could once get those three pyramids into my head. I have my book in that corner up at the ruined house. I believe we have time to look it up before you must go home. Come up, Dobbin!” The willing old horse strode forward again. “It’s not just geometry that bothers me,” Elizabeth confessed. “We had some questions in history to-day and it frightened me to have them show how little I knew. We were asked who were the Barbary pirates and what was the greatest time of America’s merchant marine. Those are just the things I never can remember.” “History doesn’t seem so hard,” returned David, “except that, if you study it without a teacher, you get so interested in some parts that you forget to pay any attention to the others. You say that they asked you about the merchant marine and America’s ships? Now I never thought of paying any attention to that. Hurry, Dobbin. I begin to think that we have no time to lose.” They turned into Somerset Lane, hastened up the final slope and left the white horse tied to the cottage gate. Miss Miranda seemed to be still at work in the garden, so they deposited Dick and the package of eggs in the kitchen and went scurrying across the lawn to the gate in the wall. If they were to vanquish their common enemy before dark it was necessary to make some speed.