purpose. For the glass, being once in adjustment, is only thrown out of
position by the rotation of the earth, and if the glass is rotated
sufficiently about its own axis, in a direction opposite to that of the
earth, it will retain its adjustment. In order to have the adjustment
equally good when the sun was either north or south of the equatorial
plane, it was sufficient to mount a secondary axis upon the primary one
and at right angles to it. About this the glass could be turned through
an angle of 23½°, either way, from the position which it should have
when the sun was in the equatorial plane.

[Illustration: BLUE PROCESS PRINTING APPARATUS.]

_The Construction of the Adjusting Mechanism._--I desired to have the
mechanism as compact and inexpensive as possible, and to have the frame
well balanced about the primary axis, in every position. I also desired
to have a rotation of nearly 180° about the principal axis. The plan
adopted will be most easily understood by referring to the drawing which
illustrates it. The axes are composed chiefly of wood. They are built up
from strips which are 3 inches × 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