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The Catapult
wound down by the roller. To prevent this slipping of the arm its butt-end
should be slightly increased in bulk, as shown in fig. 193, p. 280.
The cup or circular hollow at the end of the arm - in which the stone
is laid - is 5 in. wide and 2 in. deep at its centre.
The arm should be tightly bound at short intervals with lashings of
quarter-inch cord, fig. 196, page 285. Sometimes an arm will endure the
great strain applied to it from the first and show no sign of fracture,
though it may bend not a little when, it is wound down to its full extent.
It is, however, probable that the first arm or two tried in the catapult
will give way, especially if too much initial pressure is put upon them.
The arm should be tested by degrees and only pulled down its full distance
after several trials at shorter ones.
The ancients had the same difficulty in obtaining arms for their large
catapults that I have experienced with smaller ones.1
For this reason their engineers constructed the arm of a catapult of
three longitudinal pieces.
They first fastened three smooth and closely fitting planks together
with glue and with small rivets; then they shaped the planks, thus held
together, into an arm of correct size and outline.
The arm, except its enlarged head-end, was next wrapped tightly round
its entire length with several layers, one above the other, of strong linen
soaked in glue, the linen being cut in strips about 3 in. wide.
Finally strong cord, also soaked in glue, was closely lashed over the
linen from the butt-end of the arm to the cup for the stone.
The arm was made on the same principle as a carriage spring, or a longbow
of several pieces, and was infinitely stronger and more elastic than one
formed of solid wood.
The Winches of the Catapult, Fig. 197. For Dimensions See Next Page
These are the most important parts of the catapult, and generate its
projectile force.
However carefully a catapult may be constructed, its effectiveness chiefly
depends upon the two winches that twist the skein of cord in which its
arm works.
The plans in fig. 197 show a winch and its cross-bar in various positions.
1 I smashed six arms in succession in the first
fairly large catapult I made before I could obtain one that lasted. The
piece of selected ash used by a cart-builder for cutting a pair of shafts
from can usually be made into an arm for a catapult. |
