The Fourth And Last Instrument, And
The Largest Of All, Was One Consisting As It Were Of Three Or Four Huge
Astrolabes In Juxtaposition [D]; Each Of Them Having A Diameter Of Such A
Geometrical Pace As I Have Specified.
The fiducial line, or Alhidada, as
it is called, was not lacking, nor yet the Dioptra.[10] Of these
Astrolabes, one having a tilted position in the direction of the south,
represented the equator; a second, which stood crosswise on the first, in
a north and south plane, the Father took for a meridian; but it could be
turned round on its axis; a third stood in the meridian plane with its
axis perpendicular, and seemed to stand for a vertical circle; but this
also could be turned round so as to show any vertical whatever. Moreover
all these were graduated, and the degrees marked by prominent studs of
iron, so that in the night the graduation could be read by the touch
without a light. All this compound astrolabe instrument was erected on a
level marble platform with channels round it for levelling. On each of
these instruments explanations of everything were given in Chinese
characters; and there were also engraved the 24 zodiacal constellations
which answer to our 12 signs, 2 to each.[11] There was, however, one error
common to all the instruments, viz. that, in all, the elevation of the
Pole was assumed to be 36 deg.. Now there can be no question about the fact
that the city of Nanking lies in lat.
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