_Mrs. B._ I believe I must make a sketch of you and your brother riding on a plank, in order to convince you of your error. (fig. 4. plate 4.) You may now observe that a lever can move only round the fulcrum, since that is the centre of motion; it would be impossible for you to rise perpendicularly, to the point A; or for your brother to descend in a straight line, to the point B; you must in rising, and he in descending, describe arcs of your respective circles. This drawing shows you also how much superior his velocity must be to yours; for if you could swing quite round, you would each complete your respective circles, in the same time. _Caroline._ My brother's circle being much the largest, he must undoubtedly move the quickest. _Mrs. B._ Now tell me, do you think that your brother could raise you as easily without the aid of a lever? _Caroline._ Oh no, he could not lift me off the ground. _Mrs. B._ Then I think you require no further proof of the power of a lever, since you see what it enables your brother to perform. _Caroline._ I now understand what you meant by saying, that in mechanics, velocity is opposed to weight, for it is my brother's velocity which overcomes my weight. _Mrs. B._ You may easily imagine, what enormous weights may be raised by levers of this description, for the longer, when compared with the other, that arm is to which the power is applied, the greater will be the effect produced by it; because the greater is the velocity of the power compared to that of the weight. Levers are of three kinds; in the first the fulcrum is between the power and the weight. _Caroline._ This kind then comprehends the several levers you have described. _Mrs. B._ Yes, when in levers of the first kind, the fulcrum is equally distant from the power and the weight, as in the balance, there will be an equilibrium, when the power and the weight are equal to each other; it is not then a mechanical power, for nothing can in this case be gained by velocity; the two arms of the lever being equal, the velocity of their extremities must be so likewise. The balance is therefore of no assistance as a mechanical power, although it is extremely useful in estimating the respective weights of bodies. But when (fig. 5.) the fulcrum F of a lever is not equally distant from the power and the weight, and the power P acts at the extremity of the longest arm, it may be less than the weight W; its deficiency being compensated by its superior velocity, as we observed in the _see-saw_. _Emily._ Then when we want to lift a great weight, we must fasten it to the shortest arm of a lever, and apply our strength to the longest arm?