They are built up
from strips which are 3 inches x 7/8 inch, and from small pieces of 2
inch plank. They are stiffly braced. A pair of ordinary hinges permit
the secondary rotation to occur, while a pair of cast iron dowel pins
with their sockets, such as are used in foundry flasks, serve as pivots
during the primary rotation.
_The Adjustments._--The adjustment about the secondary axis does not
need to be made more frequently than once a week, or once a fortnight.
In order to prevent rotation about this axis when in adjustment, two
cords lead from points which are beneath the back board, and as far
removed from the secondary axis as is convenient. Each cord passes
forward and backward through four parallel holes in a wooden block which
is attached to the primary axis. The cords can be easily slipped in the
holes by pulling their loops, but the friction is so great that they
cannot be slipped by pulling at either end. It takes about twice as long
to make the adjustment as would be necessary if a more expensive device
had been used; but this device is at once so cheap, so secure, and has
so seldom to be used, that it was thought to be best adapted for the
purpose. To prevent rotation from occurring about the primary axis when
it is not desired, a bar parallel to the secondary axis is attached by
its middle point to the primary axis near one end.
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