It
sometimes happens that when foot-passengers are knocked down by
carriages, the wheels pass over them with scarcely any injury,
though, if the weight of the carriage had rested on their body,
even for a few seconds, it would have crushed them to death. If
the view above taken is correct, the injury in such circumstances
will chiefly happen to that part of the body which is struck by
the advancing wheel.
41. An operation in which rapidity is of essential importance
is in bringing the produce of mines up to the surface. The shafts
through which the produce is raised are sunk at a very great
expense, and it is, of course, desirable to sink as few of them
as possible. The matter to be extracted is therefore raised by
steam-engines with considerable, and without this many of our
mines could not be worked velocity, with profit.
42. The effect of great velocity in modifying the form of a
cohesive substance is beautifully shown in the process for making
window glass, termed "flashing", which is one of the most striking
operations in our domestic arts. A workman having dipped his iron
tube into the glass pot, and loaded it with several pounds of the
melted "metal", blows out a large globe, which is connected with
his rod by a short thick hollow neck.
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