“When I was a boy I went down to the wharf with another boy to see the first iron steamship that had ever crossed the ocean to Philadelphia. Now, hearing of an iron steamship seemed to us then a good deal like hearing of a leaden kite or a wooden cooking-stove. But we had not been long aboard of her, before my comrade said in a tone of contemptuous disgust: “Pooh! I see how it is. She’s all lined with wood; that’s the reason she floats.” I could not controvert him for the moment, but I was not satisfied, and sitting down on the wharf when he left me, I set to work trying mental experiments. If it was the wood inside of her that made her float, then the more wood the higher she would float; and, mentally, I loaded her up with wood. But, as I was familiar with the process of making boats out of blocks of wood, I at once saw that, instead of floating higher, she would sink deeper. Then, I mentally took all the wood out of her, as we dug out our wooden boats, and saw that thus lightened she would float higher still. Then, in imagination, I jammed a hole in her, and saw that the water would run in and she would sink, as did our wooden boats when ballasted with leaden keels. And, thus I saw, as clearly as though I could have actually made these experiments with the steamer, that it was not the wooden lining that made her float, but her hollowness, or, as I would now phrase it, her displacement of water.   In such ways as this, with which we are all familiar, we can isolate, analyse or combine economic principles, and, by extending or diminishing the scale of propositions, either subject them to inspection through a mental magnifying glass or bring a larger field into view. And this each one can do for himself. In the inquiry upon which we are about to enter, all I ask of the reader is that he shall in nothing trust to me.”