“Whether you believe or not that the Lydians invented sports shall not be a subject of contest between us,” said the reverend antiquary; “but,” continued he, “one thing is quite certain, that the Lydian games were at first called _Lydi_ by the Romans; and afterwards, by corruption, _Ludi_; a presumption I must needs say in favour of the Lydian claim; but enough of this: to what do you propose we should next turn our attention? I doubt not you have some new sport for our recreation as well as our instruction,” added the vicar. “We will, if you please, attend the children to their _see-saw_, which the gardener has lately constructed for them,” said Mr. Seymour. The party accordingly walked to the grove, in which a plank had been placed across a wooden post: and upon which Tom and John had been riding for some time in the earlier part of the morning. The boys again mounted their new hobby; and, after amusing themselves for some minutes, Mr. Seymour desired them to stop, in order that Tom might explain the principle upon which the _see-saw_ acted. Tom replied, that he was not aware of any principle which could apply to riding on a plank. “Have I not often told you, my dear boy, that the principles of Natural Philosophy may be brought to bear on the most trivial acts of life? Listen, therefore, and you shall find that your present amusement teems with instruction. You are already well acquainted with the nature and operations of the centre of gravity; tell me, therefore, whereabouts it lies in the plank upon which you are riding.” “I should think,” replied Tom, “that in this instance, the centres of gravity and magnitude must coincide, or be very nearly in the same point.” “The centre of gravity must, as you say, be very nearly in the middle of the board; and if that be the case, you will allow that, supposing those who ride upon it are of equal weight, the plank must be supported in the centre to make the two arms equal; but you and John are of unequal weight, so that you perceive the plank must be drawn a little farther over the prop to make the arms unequal; and John, who is the lightest, must be placed at the extremity of the largest arm. Thus arranged, you will exactly balance each other; and as each of you, on your descent, touches the ground with your feet, the reaction affords you a spring, which destroys the equilibrium, and enables you to oscillate in arcs about the centre of motion.” [Illustration: Two children on a see-saw, with circles drawn around them to show their motion.] “Do we then describe the arcs of a circle as we ascend and descend?”