“Undoubtedly you must. Look at this diagram,” said Mr. Seymour, “and you will see at once that the plank can only move round its centre of motion; for how could you rise, or your brother fall, perpendicularly in a straight line? You must, in rising, and he, in descending, describe arcs of your respective circles. It is equally evident that his velocity must be very superior to yours; for, if you could swing quite round, you would each complete your respective circles in the same time.” “It would really appear so,” said Tom; “and I have myself observed, that the lighter person has the better ride, as he moves both farther and quicker, and I now understand the reason of it; it is because being farther from the centre of motion, he describes a larger arc.” “The greater velocity with which your little brother moves, renders his momentum equal to yours. You have the most gravity, he the greatest velocity; so that, upon the whole, your momenta are equal: for you, no doubt, remember that momentum is weight multiplied into velocity.[33] You have here then a striking instance of mechanical advantage gained by opposing motion to matter, or velocity to weight; for I think you will readily admit, that without the aid of the plank, your little brother could never have raised you from the ground.” “That is clear enough,” said Tom. “The plank, then, thus arranged,” continued his father, “constitutes what has been termed a _mechanical power_, to which the name of _lever_ has been given; it is not, however, my intention at present to enter into the history of these powers, of which there are six distinct kinds; the one presented to you, in the instance of the _see-saw_, is perhaps the most simple, and not the least important of them.” “It is very curious,” observed the vicar, “to reflect upon what a simple, and apparently trifling fact, the powers of civilized man may be said to depend. The single truth you have just announced, of making velocity a compensation for weight, has supplied his weak arm with the means of controlling the very elements.”(23) “It is very true,” said Mr. Seymour, “and we might go so far as to say that, had it been the will of the Almighty Creator of the universe to have withheld from matter that property which we have been discussing, man must have remained the most helpless and forlorn of his creatures. I now propose,” added Mr. Seymour, “to accompany the children to their swing; the present is a suitable opportunity for giving them some idea of the doctrine of oscillation, or the theory of the pendulum.” “Let us proceed, then, to the _Icarian Game_,” exclaimed the vicar.