To accomplish this, a helm was attached to the lower
division of the main conductor, by which the latter could be made to
move at will in any direction within the limit of its rotation.
Controlling this helm was, in the open or steering circle on the right
hand, a small knob to be moved exactly parallel to the deviation of
the star in the mirror of the metacompass. The left-hand circle, or
discometer, was divided by nineteen hundred and twenty concentric
circles, equidistant from each other. The outermost, about twice as
far from the centre as from the external edge of the mirror, was
exactly equal to the Sun's circumference when presenting the largest
disc he ever shows to an observer on Earth. Each inner circle
corresponded to a diameter reduced by one second. By means of a
vernier or eye-piece, the diameter of the Sun could be read off the
discometer, and from his diameter my distance could be accurately
calculated. On the further side of the machinery was a chamber for the
decomposition of the carbonic acid, through which the air was driven
by a fan. This fan itself was worked by a horizontal wheel with two
projecting squares of antapergic metal, against each of which, as it
reached a certain point, a very small stream of repulsive force was
directed from the apergion, keeping the wheel in constant and rapid
motion.
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