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stronger structure, because they are pervious; that is, their
feathers are separate and the air passes through them. But the bat
is aided by the web that connects the whole and is not pervious.
1
124.
TO ESCAPE THE PERIL OF DESTRUCTION.
Destruction to such a machine may occur in two ways; of which the
first is the breaking of the machine. The second would be when the
machine should turn on its edge or nearly on its edge, because it
ought always to descend in a highly oblique direction, and almost
exactly balanced on its centre. As regards the first--the breaking
of the machine--, that may be prevented by making it as strong as
possible; and in whichever direction it may tend to turn over, one
centre must be very far from the other; that is, in a machine 30
braccia long the centres must be 4 braccia one from the other.
[Footnote: Compare No. 1428.]
1
125.
Bags by which a man falling from a height of 6 braccia may avoid
hurting himself, by a fall whether into water or on the ground; and
these bags, strung together like a rosary, are to be fixed on one's
back.
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