“I see,” said Louisa, “that all you have said about the woman and her pail must be true; but how could she have learned the principle which thus enabled her to keep the centre of gravity in its proper place?” “By experience. It is very unlikely that she should ever have heard of such a principle, any more than those people who pack carts and waggons, and yet make up their loads with such accuracy as always to keep the line of direction in, or near, the middle of the base. But to proceed to another example--have I not frequently cautioned you against jumping up suddenly in a boat? Can you tell me upon what principle such an operation must be attended with danger?” “I suppose,” said Tom, “for the very same reason that a waggon is more likely to be overturned when its top is too heavily laden; it would elevate the centre of gravity, and thereby render the line of direction liable to be thrown beyond the base, and so upset the boat.” Mr. Seymour observed, that after this lesson he thought the balancing which Tom and Louisa had witnessed at Astley’s theatre, last year, would cease to appear so miraculous. Louisa declared that she had now discovered the whole mystery. “You have doubtless perceived,” said her father, “that the art entirely consists in dexterously altering the centre of gravity upon every new position of the body, so as constantly to preserve the line of direction within the base. Rope-dancers effect this by means of a long pole, the ends of which are loaded by weights, and which they hold across the rope. If you had paid sufficient attention to their movements, you must have perceived how steadily they fixed their eyes on some object near the rope, so as to discover the slightest deviation of their centre of gravity to one or the other of its sides, which they no sooner detect, than they instantly rectify it by a countervailing motion of their pole, and are thus enabled to preserve the line of direction within the narrow base. This very same expedient is frequently practised by ourselves; if we slip or stumble with one foot, we naturally extend the opposite arm, making the same use of it as the rope-dancer does of his pole. Many birds, also, by means of their flexible necks, vary the position of their centre of gravity in the same manner. When they sleep, they turn it towards the back, and place it under the wing, in order to lay the greatest weight on the point above the feet.” “What an interesting subject this is,” cried Louisa, “and how many curious things it is capable of explaining!” “Indeed is it; and I shall take an opportunity of pointing out several specimens of art (9) which are indebted for their stability to the scientific application of the principle we have been considering;--but I have now a paradox for you, Tom.” “Let us hear it, papa.”