The small balls of sewing
cotton, so cheap and so beautifully wound, are formed by a
machine on the same principle, and but a few steps more
complicated.
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.
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