It was
evident that when the sun was in the equatorial plane, the surface of
the glass should contain a line which was parallel to the axis of the
earth; and further, that if such a glass was firmly attached to an axis
which was parallel to that of the earth, it would fulfill the desired
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 231/2 deg., 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 deg. 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.
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