36. In turning from the smaller instruments in frequent use to the larger and more important machines, the economy arising from the increase of velocity becomes more striking. In converting cast into wrought-iron, a mass of metal, of about a hundredweight, is heated almost to white heat, and placed under a heavy hammer moved by water or steam power. This is raised by a projection on a revolving axis; and if the hammer derived its momentum only from the space through which it fell, it would require a considerably greater time to give a blow. But as it is important that the softened mass of red-hot iron should receive as many blows as possible before it cools, the form of the cam or projection on the axis is such, that the hammer, instead of being lifted to a small height, is thrown up with a jerk, and almost the instant after it strikes against a large beam, which acts as a powerful spring, and drives it down on the iron with such velocity that by these means about double the number of strokes can be made in a given time. In the smaller tilt-hammers, this is carried still further by striking the tail of the tilt-hammer forcibly against a small steel anvil, it rebounds with such velocity, that from three to five hundred strokes are made in a minute. In the manufacture of anchors, an art in which a similar contrivance is of still greater importance, it has only been recently applied. 37. In the manufacture of scythes, the length of the blade renders it necessary that the workman should move readily, so as to bring every part of it on the anvil in quick succession. This is effected by placing him in a seat suspended by ropes from the ceiling: so that he is enabled, with little bodily exertion, to vary his distance, by pressing his feet against the block which supports the anvil, or against the floor. 38. An increase of velocity is sometimes necessary to render operations possible: thus a person may skate with great rapidity over ice which would not support his weight if he moved over it more slowly. This arises from the fact, that time is requisite for producing the fracture of the ice: as soon as the weight of the skater begins to act on any point, the ice, supported by the water, bends slowly under him; but if the skater's velocity is considerable, he has passed off from the spot which was loaded before the bending has reached the point which would cause the ice to break. 39. An effect not very different from this might take place if very great velocity were communicated to boats. Let us suppose a flatbottomed boat, whose bow forms an inclined plane with the bottom, at rest in still water. If we imagine some very great force suddenly to propel this boat, the inclination of the plane at the forepart would cause it to rise in the water; and if the force were excessive, it might even rise out of the water, and advance, by a series of leaps, like a piece of slate or an oyster shell, thrown as a 'duck and drake'.